Want some ideas outside Small Taxonomy Pools? Want a different link to animalTropes? Want to read about some creatures you’ve never heard of? Make up a new alien species that makes some sense? You’ve come to the right place.
Some Basics
When biologists started classifying life forms, everything was divided between animals and plants. Then single celled organisms were found that crossed boundaries, they could both move and photosynthesize, so microbes got their own c=group. Then biologists realized that fungi also had a mix of plant and animal like features, so they also got their own group. Microbes were further split.
Today, scientists use phylogenetic classification, a group includes everything with a particular common ancestor. Most likely, the common ancestor isn’t known, so scientists will use several techniques to figure out which groups are more closely related, these groups should have some ancestor in common, and are grouped together. The closest non animal relative to animals is a group called choanoflagellates, so animals are everything more closely related to cats, lizards, T-rex, or any other creature you like than to choanoflagellates.
But the group was originally based on physical features, and all animals have certain ones in common.
- Animals are Eukaryotes. All earth life is made of cells (apart from viruses, if you include them as life), some cells have lots of mini organs including a nucleus that stores genes, these are Eukaryotes. Eukaryotes also include fungi, algae, plants, many kinds of microbes, but not bacteria, or a group called Archea that has some chemical differences to bacteria.
- Animals are muticellular. Like plants, fungi, and some algae, animal bodies are made of lots of cells. These cells specialize, changing shape and other properties to fill specialized roles in the body. Most animals have distinct tissues, which combine similar cells into a larger structure,
- Animals need food: The word for needing to eat other living organisms, or artificial imitations, is heterotroph. Plants, algae, and some bacteria and archea are autrophs, they can use nonliving sources of energy such as the sun to power themselves. All animals need to eat things is the result.
- Animals need oxygen: Aerobic is the word for this. Some organism can break down material with no oxygen to get enough energy. Some animals can do this for short bursts, like your muscles when sprinting, but outside those short periods animals need oxygen for energy.
- Animal cells lack cell walls: Plants, fungi, and many bacteria have a rigid structure surrounding their cells. This provides support in plants and fungi, helping them stand up. Animals lack this, having flexible cells instead. This allows animals to move, which almost no other multicellular organisms can do quickly, movement is probably the most obvious animal quality for most people.
The closest relatives of animals are a group of microbes called choanoflagellates. Their cells have a tail for propulsion, are rounded, a few specialized animal cells in some groups resemble them. Fungi are close relatives of animals, as you might guess by mushroom’s close to meat like taste and texture. Fungi share a number of chemicals in common including pigments like melanin, and chitin in their cell structure.], a material found in arthropods in particular. Plants, slime molds, and algae are not that closely related.
See Evolution for more.
Officially, biologists use a system called phylogenetic classification: a group includes all descendants of some common ancestor. To classify animals, biologists need to know how animals are related. In practice, the exact ancestor is almost never known, but a tree of relationships between different types of animals can be mostly reconstructed, groups are defined by all branches coming from a node or other place on the tree.
Thanks to how evolution works, this system mostly matches common names of animals, One exception is when one group evolves from within another, such as land vertebrates evolving from fish or bees evolving from wasps, technically “fish” isn’t a group and wasps include bees, but a scientists won’t assume a bee stung you while you ate chicken if you say that a wasp stung you while you ate fish. Another possibility is unrelated groups evolving similar characteristics and being lumped together. Outside of animals, algae are like this, three separate groups have evolved but they are similar enough to be called the same thing.
To figure out how life is related, including animals, scientists look for certain types of similarities, preferably ones that are difficult to change through evolution, or are more or less random and don’t affect survival much. Physical structures might use shell or bone structure if the animal has one, or structures of organs where multiple forms can work well. Genetic studies use mutations, often in noncoding DNA or genes with minor effects, to measure the time between splits and similarity between animals; these assume that mutations are more or less random, and when groups split a different collection of mutations will build up over time.
Characteristics that do affect survival a lot and evolve easily will likely adapt quickly to a role or environment, meaning unrelated animals living in a similar way will have them in common while related animals that will live differently will not. The shape of whales, fish, and prehistoric ichthyosaurs is an example; it is well adapted to moving through water, but other features such as bone structure, whether the creature has gills or lungs, and evolutionary intermediates point to whales and ichthyosaurs having land based ancestors.
Prehistoric animals obviously can’t have genes sampled, but physical characteristics from fossils can be compared. If there are enough fossils of the right animals, ancestry can be directly traced. Fossilization is a chancy process, a dead animal needs to be under the right conditions to successfully fossilize. Having hard body parts helps a lot, vertebrates, molluscs, brachiopods, coral are well understood thanks to shell, bone, tooth, and coral structure fossils, many soft bodied or very small animals have nothing or next to nothing.
The first animals are visible in fossils around 660 million years ago. Some studies suggest they existed around a billion years ago, some more recent, but no hard evidence narrows this down. Multicellular life in general was developing at this time. The first animal fossils look like fixed in place creatures that fed on mats of bacteria on the ocean bottom, possibly with some filter feeding. Sponges seem to be the only survivors of this type of animal. Sometime around 550-600 million years ago, some animals developed movement. The first fossils of burrows developed around 540 million years ago, around the same time animal diversity increased enormously in something called the cambrian explosion. Most modern animal groups came into existence around this time. Prehistoric Life has you covered from here.
This is not how scientists see it, as Evolution describes. While predators and large animals are individually threatening, collectively prey and smaller animals do quite well for themselves. Animals are usually spread out enough that animals aren’t constantly under attack, as you’ll notice if you go hiking or spending time in a wild area, so most smaller animals and prey spend most of their time going about their business and occasionally running or hiding from something dangerous. Lions, Tigers, and Bears might be the big predators in modern environments, but plenty of smaller cats, lizards, birds, spiders, worms, etc. are living out their lives quite successfully. Likely this is true of dinosaurs and mammals, mesozoic mammals were filling similar roles to racoons, rats, squirrels, etc. that live in equivalent areas today.
As for higher and lower animals, The evolution link describes opinion well. All animals have had to face a similar set of challenges in living in an environment, finding food, etc., so anything that survives that is well evolved and adapted to its role: Sponges and jellyfish have adapted just as much as cuttlefish, bees, or whales. Complexity involves tradeoffs, more energy, more difficult growth and development are the big ones, so a simpler and less active animal may well work better in certain roles. Vertebrates scientifically are seen as just one group of animals, so invertebrate makes as much sense as nonarthropod or isntmollusc or other-than-cnidarian, though on sites like this one invertebrate gets some use as vertebrates are more more represented in pop culture and other areas. If an animal form has lasted a long time, it means it was very well adapted for an environment, and that said environment has also lasted a long time, so “primitive” animals are those which ended up with a long lasting, useful shape and way of living. "Backward" evolution, where an animal ancestor gained something and lost it again has happened often: Flightless ants and ostriches had ancestors who evolved and lost flying, whale ancestors moved from sea to land and back, slug and squuid ancestors gained and lost shells, flatworm ancestors gained and lost many internal organs.
Animals are just living their lives, not pursuing any larger goals, but population, diversity in species or in roles a group fills, geographic spread, amount of time it has existed before going extinct, are all used as measures of success.
Technical specs a.k.a. Animal characteristics
Colors, shapes, sizes, what it eats, how it acts, lots of things make animals different and cool. Some characteristics are described in sections below.
Like a lot of human built machines, animal characteristics are tradeoffs, with survival/success based on how well a combination of qualities fits together. Most commonly, the tradeoff is size and greater nutrition needs, any extra tooth, brain, poison, general size, etc. takes energy to grow and maintain, meaning the animal needs more food. Animal evolution could be thought of as a random, non-top down designed engineering challenge, and animals often face similar issues as human machines, with roughly similar results: flying animals use airfoils and produce a lot of power for their size with metabolism and flight muscles, similar to airfoil using flying machines with high power to weight engines. The streamlined fish shape isn’t too far off from a submarine or ship form, and waterjet vs. propeller is similar to squid jet vs. fish tail (fast but power hungry vs. usually slower but more efficient). Some animal characteristics have inspired machine designs, such as neural net programs based roughly on how people learn, and lots of unsteady state motions, especially fluid flow (think eel or snake swimming or insect flying) are used by animals but not understood well, they may have future applications.
Deep Sea, Desert, Deciduous trees, …uh..dirt? …running out of D’s. Where an animal lives affects everything else about it. Life lives in a very narrow band near the earth’s surface,from the dirt/seafloor below ocean trenches and some deep mines, to flying high in the atmosphere, with much of it in the oceans or close to ground level. This narrow band has very diverse environments, however. Where to live is also affected by size, a microscopic animal will find plenty of space in a body or tiny hole, for example, which to a large animal would just be a piece of the environment. Richer areas can support more life, including more animals and a big variety of food sources, but this means more competition.
Animals are usually well adapted to one environment that they live in long term, but short trips to others can be common, such as land animals swimming or ground dwelling animals climbing. These can be a useful part of an animal’s lifestyle if it regularly lives near a border area. How strongly the boundary restricts an animal depends on the animal’s adaptations, and how the animal is built. Water vs. Land is a very strong boundary, saltwater vs. freshwater, crossing mountains, or going from a desert to a lush area, are also quite restrictive. Something like forest to grassland, or going between layers of an ocean, are much weaker barriers to animal movement.\\
Some types of environments:
* Water vs. Land: Water and air are very different materials, very few animals can live in both. The differences affects almost everything an animal does. Breathing is probably the most obvious, but water also supports animals in a way air does not, which allows 3-D movement, It can diffuse wastes away, it affects sight, smell, and hearing, it changes temperature less than air does, among other differences.
* Saltwater vs Freshwater: You know how you shouldn’t drink too much seawater, because you’ll lose more water getting rid of the salt than you took in? Animals living in water must handle this all the time. A concentration difference between water inside and outside an animal means water will either diffuse in or out, and an animal will need to either expel extra water, or extra salts or other material to keep its internal fluids concentrated the right amount. Animals usually are adapted with appropriate systems for a certain salinity range, go outside and the animal dies from too much or too little water.
* Richer vs. Poorer environments; All animals need to eat something, and eventually that eating leads to other life forms that get energy and nutrients from the external environment. To have more life in an area, an environment needs nutrients, a source of energy, almost always sunlight, water, and avoid extremes that can kill most life. Environments with all these will be very productive and support a lot of life, environments missing something a lot less.
Shallow water, rainforests, and weird deep sea hydrothermal systems are examples of highly productive areas. All have nutrients from soil/the ground or the sea floor, the first two have lots of sunlight for energy while hydrothermal vents have heat and heat driven chemistry, rainforests have lots of rain while the other areas are in water. Deserts and most of the ocean are examples of poorer environments. Deserts are missing water, the deep ocean has little sunlight and relies on falling material from above for energy (apart from hydrothermal vents and some other seeps), and the open ocean surface has few mineral nutrients, being far away from any rocks that they come from.
Richer environments can support more of all kinds of life, including animals. This greater number allows for lots of different species to maintain good sized breeding populations, meaning more species variety. However, this also means more competition for space. Poorer environments are often slower paced, animals need to conserve limited resources and adapt to low populations.
* Complex terrain: Grasslands, open ocean, deserts are easy to navigate, wide open spaces. Forests, reefs, and rugged terrain like mountains are quite a bit more complicated, with spaces to hide in, roundabout travel routes with obstacles, and changes from location to location. Animals in more complex environments may need unique travel methods, and some way to navigate or otherwise find what they need. In more open environments, fast travel can be useful, since there is room to move quickly.
* Temperature, Wind, Moisture, Dissolved sediments, other environmental properties: Obviously, animals will need to adapt to these. They will also need to adapt if these conditions are variable, such as seasons or big temperature changes between day and night, which require an ability to handle a variety of conditions.
* Larger living things: Some animals live on other animals or plants. Some live Inside other animals, or burrow into plants. Living things for obvious reasons don’t want to be burrowed into, and carrying around extra weight isn’t great either, so such animals need to avoid whatever defenses are there. Plants tend not to have many of these, but animals can scrape, scratch, or bite off those living on them, and anything inside has to fight an immune system.
Living inside an animal gives good temperature control and a good source of food. Almost all animals living in others are parasites. Living on the outside of animals might just be a good attachment point, it also may offer feeding opportunities on blood, skin flakes, other parasites, or such. Living on or in plants may be useful for parasites, or the plant is just an easy surface to travel on or burrow into, as with some creatures living in wood.
* Burrowing: Burrowing through sediments takes more effort than swimming or moving through air, and there is little light, plus sound and smells don’t travel that well. In return, underground provides protection from things at the surface, including temperature changes and and sediments contain plant routes and other accumulated debris for nutrients. Cave dwelling animals let something else do the burrowing, but still have to handle the lack of light and other underground conditions.
Among multicellular life, animals are unique in having no cell wall, a rigid structure surrounding cells in plants and fungi. This removes a source of support and structure from animal bodies, but allows flexibility, which allows movement. The first animals didn’t have moving parts, and today’s sponges still don’t. The vast majority, however, can move some part of their bodies, and the vast majority of those can move their whole bodies from place to place.
In almost all animals, movement comes from muscles. Muscles can only apply force by contracting, they work using interlocking rows of proteins, where when relaxed the proteins are spread out, but with energy and a signal they slide into each other’s gaps and pull together. Here’s a video
of how this works for humans. Because muscles can only contract, they must be arranged in groups to provide force in several directions, and often have something else to work against. In addition to body part movement, muscles also pump things as in a heart or movement through a digestive system, by contracting to apply pressure to whatever is being moved.
Most animal tissue is effectively bags of gelatin contained by membranes and sometimes connected by other structures (Gelatin itself is made from several structural proteins from animals). If you fill a water ballon or plastic bag with a gel, it supports itself decently well. Many types of animals are entirely soft bodied, small enough animals can support themselves in this way, as can water animals using buoyancy for support. Harder body parts trade off the energy required to make them, weight, and some flexibility for a few advantages. They can support an animal if it is standing or walking, very useful on land, or stop an animal from flopping or being blown around against wind or a strong water current. They produce something for muscles to pull against, some soft bodied organisms use tubes of fluid that muscle squeezes for a similar role. Hard body parts can push against outside materials more effectively than solid parts, such as a flapping wing moving air or a tail used for swimming in water. If positioned right, they can provide protection.
How fast and efficiently an animal needs to move depends on its lifestyle. Animals move to get resources like food or mating, or to avoid dangers such as predators or bad environmental conditions. If an animal has other ways to get these, it can save energy and get away with slow movement, with a body shape adapted to other things. A sponge or crinoid that eats ocean particles brought by a current, reproduces by throwing reproductive cells into those same currents where they can meet, has some hard body parts that are hard to eat to deter predators, and lives in relatively constant oceans, doesn’t need to move at all. A shark in the open ocean that has to cover long distances to find food and reproduce, and eats other animals that it must fight and can escape, has to move quite a bit more efficiently. Another tradeoff is immediate speed vs. endurance, analogous to sprinting vs. long distance running. Long distance travel might require more endurance, while ambushing would require more immediate speed, for a couple examples.
Movement has had more of an effect on animal body shapes than just about any other factor. Here’s a list of how movement tends to work in different environments:
* Water: Buoyancy provides support to animal bodies, water is about as dense as animal flesh, so animals can easily stay above the seabed. Water is also heavier and more viscous than air, meaning moving through water takes a lot more energy for the same speed as air, but pushing against water produces a lot more force than pushing against air, assisting swimming. Slower moving animal can push using any flexible body part against the water, or against the seafloor to walk on it. To move faster, drag reduction is very important, so fast moving animals usually have long, thin shapes as seen in most fish and squid. For fluid mechanics reasons, propulsion using fins or tails of the right shape are more powerful and energy efficient, a flat tail pushing against water is common as this is an energy efficient (for animals) propulsion method. Cephalopods uniquely have water jets, which provide lots of thrust but also take lots of energy, useful for escapes and catching things, though they swim with fins when this speed burst isn’t needed. Fixed in place animals all live in water, because water currents can transport the resources they need.
* Land: Air produces far less drag than anything else animals might move through, but also doesn’t support them. The land surface provides something to push off of, and support, but also a lot of friction if an animal tries to move across it. Crawling song the ground is fine if an animal moves slowly, as with snails, or snakes. To move faster or more efficiently, legs are extremely valuable, they reduce contact with the ground, plant on the ground instead of sliding which reduces friction losses, and give better leverage for muscles elsewhere to push off the ground. Any motion in an animal’s body means energy losses, so more efficient animals will have elastic body parts to store and release walking and running energy, while reducing unneeded body motions as much as possible. Many animals have different gaits, such as walking and running in humans or gallop, canter, trot, walking in horses. These move at different speeds based on what legs can physically do, with faster ones almost always less energy efficient than slower ones. The lack of support in air means falls can be dangerous, especially when moving fast, so balance is highly important.
In complex environments (forests, thickets, crags, cliffs, etc.) animals can’t just swim through or over them as in water, and falls are much more dangerous. Special movement methods and a good grip are necessary for these, such as primates or sloths using arms to move through trees.
* Air: Just like with human technology, heavier than air flight is difficult. Powered flight has only evolved 4-5 times, in insects and a few vertebrate groups, gliding in a few more animal groups. Animals need some way to generate lift, just like with most huma aircraft airfoils are what most use: flaps of skin, feathers, or shell provide a flat surface that can deflect and influence air as needed. Moving the airfoils allow direction control. All animals with powered flight use flapping wings, the flapping pushes air on the downstroke, then rotates to a more vertical angle which reduces friction and moves little air for the upstroke, and repeats. These also allow gliding. Powered flight requires a lot of energy to stay in the air, all vertebrate flying groups have high metabolism, smaller insects are a bit more flexible in this area. Weight reduction is also important. All groups with powered flight have hard skeletons, likely needed to keep their wings in a proper shape. While flight takes a lot of energy to stay in the air, the high speed it allows requires little energy per distance moved, plus allowing higher speed, movement over most terrain, another movement option in a complex environment, and avoiding dangers from animals at the surface. Gliding also provides some of these advantages, though for shorter periods of time, without the extra energy needed to stay in the air.
* Burrowing: In a cave or premade burrow, animals can move however they move in an open environment. Full time digging through things requires more specialized adaptations. Solid material doesn’t flow around things and must be actively moved out of the way, plus produces a lot of friction when moving over it. Burrowers as a result all move slowly. A long, thin shape is common, a smaller tunnel means less material needs to be moved for the same overall size of animal. A burrower might use short powerful limbs to move material, or body motions to push it out of the way, possibly eating material as with many worms. Animals that dig occasionally, such as digging a burrow to live in, usually don;t have as many of these adaptations, instead adapting to their main living area.
* Multiple environments: Some animals move through multiple environments regularly, such as flying animals walking along the groups. Some temporarily spend time in a different environment, such as land animals swimming or surface animals climbing. The same physics applies to these animals as applies to animals in their main environment. Swimming land animals will use their bodies or limbs to push against water, flying animals on land typically have legs to walk on. Animals that spend a lot of time in multiple environments will tend to have a mix of adaptations that work well in both, such as lobsters with tails for swimming and legs for walking in water or land, or otters with longer, thinner bodies, paddle shaped feet that can swim and walk, and a tail that works well for swimming.
All animals get energy from other living beings, which means all animals need food. A good meal for an animal gives more nutrition than it costs to get and digest, enough extra to fuel the animal’s other activities, and doesn’t poison the animal.
Animals can’t use large particles of food directly, so it must be digested down to component chemicals. All animals to do this use chemical methods to break down food, and most use physical methods as well, to break food into smaller pieces that are more easily attacked by the chemicals, than absorb the results. Catalytic proteins, enzymes, do much of the work, acidic or basic conditions also help break down a lot of nutrients. Most chemicals life uses are made into long chains of smaller units, digestion breaks down bigger chemicals into these small pieces that can be rearranged to suit the animal, or used for other purposes. Vitamins, minerals, and some other smaller chemicals are absorbable directly once dissolved. Physical methods include biting off small pieces, chewing, or stomach churning, among other methods.
Animals need a few nutrients in common. Proteins perform most functions in a body. Many types of molecules can act as energy sources, though carbohydrates and fats are more common. Animals need all required elements (Nitrogen, Calcium, etc.) supplied in some way, the major elements are common to most, but a few minor ones may vary. Water makes up body fluids, and is needed ot ensure they are properly diluted/concentrated. Interconversions are possible between many different types of molecules, but animals may be limited in what can be converted, and conversions do take energy, so some specific chemicals, like vitamins in humans, are needed. Animal’s ability to process nutrients and their diets will tend to evolve together to a point where harder to get nutrients are conserved, or methods evolve to make them from other inputs, while easier to get nutrients can be used more wastefully.
Very few foods are completely absorbed, not all chemicals in them are required or usable by the body, or possibly harmful. Leftover material that isn’t absorbed becomes dung (poop, feces, scat….toilet words are a common thing), and is expelled in some way.
You’ve probably heard of carnivores, omnivores, herbivores, and maybe scavengers, but there are quite a lot of specialized names for other types of animal diets. Piscivores eat fish, insectivores…guess, detritivores eat random particles or debris, frugivores go for fruit….It’s a long list. An animal that can use only one or a few types of food is called a specialist, an animal that eats lots of types of food a generalist. Generalist carnivores eat lots of types of meat, generalized herbivores lots of types of plants, generalist with no other word attached implies an Extreme Omnivore.
Here are some types of diets, and what they imply for the animal.
* Predators of similar sized prey: These are the most famous predators, probably. T-Rex attacking a Triceratops, crocodiles dragging large animals into the water, sharks attacking seals. On a smaller scale, squid catching fish, spiders catching larger insects, some flatworms preying on earthworms also fit this category, among many others. While successful catches are most commonly shown, in practice most attempts fail, or few attempts are made. A similar sized prey animal is a ‘’lot’’of food, just imagine a person dealing with about 100-150 ib/75 kg of meat. A person needs about 2-4 pounds of meat a day if that’s the only food item, so a carnivorous version of a human could catch something like a deer or large fish roughly every couple weeks, and accounting for inedible parts, still have have enough food for themselves plus feeding a family, or group, or just leaving it for other animals if too difficult to carry, and people need more food for our body size than most other animals. With this much food, lots of missed hunting attempts are perfectly survivable, while prey facing death is under much more pressure to escape.
Eating similar sized prey requires weapons and speed to catch and kill it, this speed is often a short burst for an ambush. If living alone, such predators need enough storage (a stomach or otherwise) to actually store a good amount of food from their catches. Digestion is meat is relatively easy, but there is often a lot at once, these predators may rest after kills to break down the meal. Meat is heavy on protein and light on carbohydrates, so animals adapt energy requirements accordingly. Some vitamins or similar chemical found in plants are also often missing, so predators will adapt to make or not require as much of them, while minerals such as calcium or sodium are more available with the opposite effect.
* Predators of smaller prey, or other prey that can’t fight back: Think anteaters or chimps eating an insect hive, baleen whales catching schools of fish, starfish attacking clams, birds eating worms, or similar. Obviously, such prey can’t physically fight back effectively, but it may still have other defenses such as shells or poison that the predator has to work around. A big challenge is finding enough food, smaller prey can often hide, and requires much total individuals eaten. Colonies or clusters are good targets for predators. Nutritionally, smaller prey and larger prey are similar.
* Scavenging: Dead meat will start to decompose, which means bacteria and fungi which may cause diseases or be poisonous to the predator if the body has been dead too long. Deaths from other causes are also random, so a scavenger needs to compensate for less stability in food sources.
In practice, much scavenging is done by meat eaters taking advantage of a dead body. Meat is meat, dead bodies don’t fight back or escape, and most animal digestion has some way to handle bacteria anyway, so meat eaters will take advantage of an easily meal if available and not too decomposed. Detritivores below also act as scavengers, dead bodies are just another type of particle or debris they can eat, especially if they can chew or separate smaller parts of a bigger corpse. The most well known pure scavengers are probably vultures and some maggots. Flying allows vultures and adult flies to cover more area and see more, allowing them to find dead bodies more easily. This also reduces the randomness of scavenging, over a wide range some sort of corpse will be available. A highly acidic stomach, and secretions by maggots, deal with microbes.
* Detritivores: Earthworms are no doubt the most well known of these. Eaters of leaf litter, skin flakes, bits of organic material in soil, or other random debris. Some appear to literally eat dirt, though they are eating for nutritious particles in the dirt, not the rocks or sand, which are passed through. These have to handle lots of different types of food, since edible debris comes in all forms, plus deal with bacteria doing the same thing. Detritus as a food source won’t try to fight, and lacks defenses compared to living beings.
* Ocean filter feeding: Sponges, Mussels, Basking and whale sharks are well known. Water can carry particles, some animals can feed on these. Many small living things also float in water currents, such as single celled algae or some very small animals. Some larger animals take advantage of this food source. These animals need a way to filter water to capture the food particles, plus some way to move lots of water through said filter. Filter feeding is a common food source for sessile animals, meaning animals that don’t move. These animals have very low metabolism, only needing energy for very basic functions, so can rely on ocean currents to do most of the work delivering food particles to them. Swimming animals that filter feed obviously need more energy, but the act of swimming will move water through their filtering systems.
* Roughage eaters, or grazers, browsers, leaf eaters, and such: Eaters of general plant material, in other words. Horses and cows eating grass, termites eating wood, leafcutter ants eating leaves….there’s a lot of these. By mass, plants are the most abundant animals on earth, so these organisms have plenty of food to eat. However, outside some specific parts, plants are poorly nutritious, and are hard to digest. The main component of plants, cellulose, can’t be digested by many animals, in human nutrition it is one of the main dietary fiber chemicals that pass through. Plants can’t fight or escape, but can have poisons, and some are physically tough or have spines, needles, or other physical defenses.
Animals that eat roughage might spend more time digesting their meal, to better exploit what is there, or use more intense digestion such as chewing. Some animals can digest cellulose, others rely on bacteria to do the job. Termites and cos are the most well known with such bacteria, since these bacteria produce methane and contribute to greenhouse warming. Large digestive systems will be common, the animal will eat more total food than a similar sized meat eater.
* Fruit, seed, tuber, and other eaters of nutritious plant parts: Think bees and hummingbirds eating nectar, fruit flies, Some parts of plants are much more nutritious than stems and leaves. Places where plants store nutrients, such as tubers, are obviously something an animal would like. Spores and seeds need to carry some starter nutrients to get the new organism going, and need to be light to spread easily, so also contain concentrated nutrients. Flowering plants have evolved to take advantage of animal eating to spread seeds, producing sugar rich nectar and fruit that animals can eat, which also spreads pollen and seeds for the plants producing them, often further than wind can do so, and in a more efficient way. Pollinators moving from flower to flower means more pollen actually fertilizes something, and seeds are spread out further, meaning less self competition and a supply of fertilizing dung to get things started.
An animal specializing in these food sources needs less total mass of food than a roughage eater, but less total food is available and the animal must search for it. These food sources are also limited in certain nutrients, which the animal must compensate for. Flowers and seeds are typically brightly colored and give up unique smells, so good senses for these things are useful.
* Fungi: Fungi aren’t common as a main food source, most fungal growth is spread out filaments called hyphae that are hard to separate and eat and most animals for the the mushroom and mold growth instead, but a lot of animals will eat fungi as part of a larger diet. Instead of cellulose, fungi use something called chitin, which is different to digest. Fungal chemistry is otherwise close to animals and not a lot have hard parts, so digestion is relatively simple to do. Fungi obviously can’t run away, but have a range of toxins, most of you have probably heard of hallucinogenic mushrooms or poisonous mushrooms.
* Body fluids, parasitism: Bloodsuckers, tapeworms and other parasitic worms, aphids eating plant juices, botflies growing in skin. When you think of parasites, this is the food source of most of them. Body fluids are an excellent nutrition source, carrying lots of ready to use nutrients for the victim organism, and packed with cells ready to be broken down for more. However, while parasites don’t kill their hosts outright, they do weaken it, so the host organism will fight back and attempt to remove the parasite. These organisms as a result need to avoid defenses, such as an immune system attack from the host, or attempts to physically remove, maybe kill the parasite. Plant fluid eaters don’t deal with as strong defenses, but may have to handle poisons. Parasites also need a way to get into the animal, piercing skin and other defenses for bloodsuckers and plant sap eaters, or getting completely inside the other organism, which means burrowing in, or entering through an existing orifice like a digestive system or wound, digestive systems in particular are obviously hostile and an animal will need protection to move through it.
Halfway between parasite and predator is something called a parasatoid, where an animal lives inside another one for a long time, but is guaranteed to eventually eats up enough of the host to kill it. Many insects use this method of feeding when larvae.
* Extreme Omnivore: Rats, Crows, household cockroaches, and most famously the species reading this page, which eats just about everything mentioned in this section except wood, grass stems (grass seeds are still fair game, grains are in this category), and random ocean particles. Being a generalist eater has many advantages. From the same territory, more potential food sources means more food available. There is less risk, since if one type of food runs out, others are available. More environments can be lived in, since the animal is likely to find something it can eat no matter where it goes. However, less specialization might mean less ability to handle one particular type of food, though many generalists do just fine anyway. Animals eating lots of different foods will seem to have mix and match body parts from several types of more specialized eaters, plus lots of general purpose adaptations that can be used multiple ways.
Many animals are actually less specialized than you might think. Fruit eaters might pick infested fruit with insects, nutritionally this gives some protein. Large herbivores occasionally will eat small animals. In between full on Omnivore and full on specialist, most animals will eat food from several categories described here: predators will scavenge when they can as described in that bullet point, many plant eaters will happily grab leaves, stem, fruit, or anything else soft enough and available on a plant.
In human culture, breathing, heartbeats, and blood are strongly associated with life, and for good reason. We talk about the bloody cost of war, the breath of life, the beating heart of something as its most important, central part. In medicine, airway, breathing, circulation are the first priorities for emergencies, the loss of any of these is immediately threatening. The same, more or less, is true of most animals, though the organs are different, hypothetical cephalopod CPR or Defibrillators would need to handle three hearts, Insect medics would never use mouth to mouth as they get oxygen from tubes in their body instead of lungs, and water creatures would pump oxygenated water over the gills instead of ventilating lungs.
All animals today require oxygen for energy. Reacting oxygen with food produces a lot more energy for the same amount of material than other plausible reactions. However, oxygen is not very soluble in water, nor does it attach well to other chemicals, so is far harder than food to store in a body, most animals as a result need a continuous supply.
Anything that enters an animal either grows the animal or comes out in some form, unless the animal can store a lifetime of deaweight chemical products, it must also remove waste. Some chemicals, like carbon dioxide, are the result of normal reactions. Some wastes are removed to keep an animal's internal conditions in a certain range, such as salt or water if an animal's fluids are too concentrated or dilute. Some are the unwanted breakdown products of other molecules, or chemicals or toxins the body must remove.
Small life forms in water can intake nutrients and expel waste through diffusion. In diffusion, random movement of molecules means concentrations tend to even over time, chemicals will tend to move from areas of high concentration to low concentration. An organism producing waste products will build them up, by consuming nutrients will lower their concentration, the higher waste concentration and lower nutrient concentrations within an organism vs. outside causes material to flow the desired direction. Organisms with membranes can use active transport, pulling material across using energy, but within free internal fluid and in water outside, diffusion controls movement.
Diffusion is faster with higher concentration differences, over smaller distances, and across wider areas, so smaller, flatter, lower metabolism animals can get a lot of what they need this way, many worms, for example, breathe through their skin. In larger, plumper, and more active animals diffusion would move nutrients to them and within than too slowly, armor, thick skins, or other outer structures also interfere with exchange. Many waste products also don’t diffuse away in air. Specialized structures are as a result used to intake nutrients and oxygen, and expel wastes, water and air will be pumped to these structures to help exchange materials, and fluid will be circulated within their bodies to move nutrients and wastes where they are needed.
You’ve probably heard that your lungs or small intestine has the areas of some number of football fields/tennis courts/large sports field of choice. This high surface area comes from lots of folds, it allows a large amount of material to quickly transfer into your body. Breathing organs and intestines in most animals are similar, a structure with lots of surface area, thin membranes to allow material to transfer, and circulating blood or body fluid if the animal has it to take up the nutrients. Other structures include gills for water breathing animals, dividing small tubes in many arthropods which air can diffuse into, or tubes within sponges which water is slowly pumped through and nutrients absorbed from. Some animals use a mix, absorbing some oxygen through skin and the rest through lungs or gills or equivalent.
Breathing in air vs. water presents different challenges, and very few animals can do both. Oxygen’s low solubility in water means that a large amount must be passed over gills, and gills must extract as much oxygen as possible. For some animals, water must be moved over gills to get enough, the trope of sharks needing to swim is an example, though not true of all sharks. Oxygen is much more available in air, to the point where some water living animals like whales and some fish can breathe occasionally and get enough air for long underwater periods, but breathing structures can be sensitive to drying out, and a lack of buoyancy means the structures must be supported some other way. Bring a water breather to air, and gills or other water based structures collapse or dry out, bring an air breather into water, and it cannot absorb enough oxygen and breathing structures are possibly damaged by the water. Creatures that can breathe in both either have two sets of structures, such as some fish with gills and lungs, or gill or tube like structures made of strong materials and protected against drying out. A few fish, and many crabs, use the second method.
Gills have a range of forms, from the fish gill style slabs of feathery material, to what look like antlers or tentacles in some species. The baglike lungs you have are a vertebrate and snail thing, air is circulated in them through body movements or specialized muscles such as your diaphragm. Insects, millipedes, and similar have passive exchange, through body tubes in insects or structures near the legs centipedes and millipedes. Arachnids have structures similar to crabs. Smaller land animals can use skin breathing.
If fluid is circulated inside a body, it can either be extracellular fluid, which for you is what fills blisters, or blood separated into a separated system. Circulation can be accomplished by body movements, a good fraction of blood in your vein is pumped this way, but specialized hearts are common. The complexity of hearts depends on the animals, from fish with a single chamber to the two gill hearts and body heart in cephalopods, or the four chamber bird, mammal, or crocodilian heart. Hearts to pump extracellular fluid is possible, insects have a system that pumps fluid through a tube to one end of the body, it then slowly moves to the other end as extracellular fluid.
Some chemicals, such as sugars, salts, or carbon dioxide, can dissolve in water and are carried this way. Insoluble molecules can be emulsified, or carried using proteins. Poorly dissolving oxygen is also carried its way, hemoglobin is the carrier molecule in vertebrates, other types of animals have different molecules, causing different colors of blood or body fluid.
Some waste products can be removed by the same structures that intake oxygen and nutrients: lung in animals with them remove carbon dioxide, and in water animals gills can remove some waste. However, animals may need to actively remove wastes faster than such diffusion would allow, or remove material absorbed from the environment, such as saltwater living animals concentrating and removing extra salt. Land animals of course need to actively remove solid and liquid waste as air cannot do this. Thus the kidney or equivalent structure. In many animals, this filters blood or body fluid, keeping only what the animal needs and excluding the rest. In humans among other animals, this waste is mixed with lots of water and leaves as a liquid, animals in drier environments or ancestors in the same often exclude paste of thick liquid instead. This waste can be separate or mixed with digestive leftovers.
What is metabolism, and why is temperature important? Everything life does comes from chemical reactions, technically, metabolism is the name for all these reactions. The common meaning gets pretty close, however. A person with fast metabolism is active, energetic, and can eat a ton without gaining weight, a high metabolism animal like a hummingbird similarly is very active and needs a lot of food. Slow metabolism people are though of assluggish, slow moving, and gain weight easily, similarly slow metabolism animals like sponges or some deep sea worms require very little food and do very little activity. Humans, like most mammals, have fast metabolism and can be quite active compared to average. Yes, even a couch potato: brain activity and staying warm uses a lot of energy.
Chemical reactions happen faster at higher temperatures, so animals maintaining a higher temperature can all else equal do more. However, a higher temperature than the environment needs to be maintained, either with extra heat generation which requires more food, or insulation which means carrying around more weight. Too extreme a temperature in either direction will damage things in an animal, protein structure changes at different temperatures, so too hot or cold and proteins will rearrange themselves and likely not function. Too cold and very dangerous freezing will occur, ice takes up more space than water and can burst things, it can’t flow, diffuse chemicals, has a different structure, and many other changes that are deadly to life. High temperatures can evaporate too much water if that is a problem, and cause burns if too hot.
You’ve probably heard about warm blooded and cold blooded. Birds and mammals have a combination of high temperature, constant temperature, and fast metabolism. Most other animals have the opposite, variable temperature that matches the environment, often colder than what mammals and birds run, and slower metabolism. The world being what it is, lots of animals blue these boundaries. Some sharks and a few other predatory, long distance swimming fish, some sea turtles, and some bees use physical activity to stay warm, adapting to operate at this high temperature and lots of activity without some adaptations of birds and mammals. Lizards and Crocodiles will bask in the sun to warm up and head to cooler places if too warm, controlling body temperature somewhat through behavior. Some mammals have more variable temperatures and can cool off a lot while resting or hibernating.
Animals have several adaptations to stay within their temperature tolerances, whatever that is. Some just have wide tolerances that match their environment, such as most ocean creatures, since water doesn’t tend to change temperature too rapidly this usually works well. Insulation is common, blubber in water, and fat, feathers, and fur on land all slow heat transfer and keep the animal warm. Evaporating water absorbs a lot of heat, a few land animals use sweat or panting, or roll in mud or water, for cooling. As mentioned above, moving between sunlit, hot areas and cool areas is common behavior. Animals can generate more heat directly if cold, as in shivering, or special tissue some have than directly burns food for heat. Posture, body shape, and size effect heating and cooling: larger animals will stay warmer due to Square-Cube Law, body size scales as the cube of how long they are, while the area to lose heat through scales as the square, more heat produced per area to lose it means large animals stay warmer. Rounder animals have less surface area also, and animals can curl up to do the same to stay warm, thinner, linger or flatter animals have lots of surface area, and animals can spread out to cool down.
An animal’s metabolism relates very closely to its lifestyle. Animals that hunt, move around a lot, fly, think a lot, or do some other activity a lot will have faster metabolism to supply the needed energy, this is especially important for flyers as high metabolism means more energy for the same weight, and saving weight is very important. Animals that can move slowly, sit around for things, or are in areas with little food will have slower metabolism, conserving energy. Though as usual there are exceptions and this trend is only a rough guide.
On the small side, some nematodes are microscopic and eat bacteria. On the large end, baleen whales and sauropods weighing several tons. Animals cover quite a big size range. Humans are among the larger animals. While people are small compared to lions and tigers and elephants, or even very similar gorillas, a human to a blue whale is about the same as a mouse to a human, and there are an enormous amount of even smaller animals than that.
Size matters in quite a lot of ways.
* Increased size obviously means more stuff: more muscle, more digestive system, bigger legs or arms. It also means more food needed to keep all this alive, and size can compound as more support systems are needed, such as a larger heart and brain needed to pump blood or process information from this larger body.
* Smaller animals can obviously fit in areas bigger animals can’t, while bigger animals can cross gaps or obstacles smaller ones can’t, which affects environments available for each to live in.
* Bigger animals all else equal win more fights: Larger animals usually have more muscle, and the larger size on its own can cause more damage whatever something hits. Smaller animals will be better at hiding and camouflage.
* The Square-Cube Law has a few effects. Larger animals have less surface area per body mass, so lose heat more more slowly than an equivalent smaller animal, which can be an advantage or disadvantage depending on the environment. On land or in air, larger animals need larger legs, wings, or other support than the same shape of smaller area, since mass goes as the cube but lift and force a muscle or bone can take goes as the area or cross section. Force on impact will be proportional to mass divided by the surface area of the hit region, so larger animals get more injured by collisions and falls.
* Fluid flow has different effects at different sizes, meaning large animal movement through water or air, is different in quality to smaller animals. Small insects and hummingbirds can however, for example, using properties of airflow which are too small for larger flyers to take advantage of. The same applies for land somewhat, differently sized animals might have different gaits. Bigger animals on land can get faster to a point where injuries and falls come into play, bigger animals anywhere tend to be able to cover more range.in any environment.
* Smaller animals can specialize more. A certain population is needed to avoid going extinct, there needs to be enough breeding partners to reproduce well, plus have some buffer if bad luck happens. Smaller animals require less resources to maintain the same number of individuals, which means they can get by on smaller food sources, more limited space, etc., and specialized lifestyles or food sources almost always provide fewer resources than more generalized ones. This also means more variety for smaller animals, and more species.
The largest animals in the world are mostly filter feeders, predators, plant eaters, or some mix of the previous, these food sources are the only ones that can supply enough food easily. Almost all are in the vertebrate group, a hard skeleton is needed to support land animals and very useful for supporting water animals, all the big invertebrates live in water. Other groups with skeletons need to molt, which slows growth and is dangerous if an animal can’t avoid attackers, or are stationary animals that don’t have the lifestyle to grow large. The largest of these, baleen whales and sauropods if you include prehistoric life, have food sources that benefit from large size: Sauropods eat roughage where a long trip through a digestive system is useful for digesting it, Bealeen whales filter feed, which is more efficient with a big mouth to grab big growths of plankton or schools of fish. Other qualities combine to encourage or allow large size: Sauropods have air filled bones which make them light for their size, and reproduce by laying lots of eggs, allowing fast population growth if they suffer a temporary die off and requiring a lower population. Baleen whales live in the ocean which can support them, and the wide open ocean allows most of the planet to act as a single breeding population for any species.
At the smallest scales are several worms, things called tardigrades, and other random worms and jellies and other assorted groups. These include detritivores mainly, including some bacteria eaters, and some parasites. The animals aren’t large enough to attack anything else. Many lack lungs or hearts or some other internal organs, the animas are small enough that nutrients and waste can diffuse fast enough through the animal to keep them healthy.
Animals fight or attack each other for a few reasons: to kill prey, to defend against predators, to help or defend offspring or members of a group, or control resources such as territory or mating opportunities.
In most fights outside of catching prey, intimidation is more useful than actual fighting. (Sun Tzu’s first principle was on to something.) Fighting takes energy, and even the winner of a fight can get injured, so if an animal can scare away a rival or attacker, so much the better. Also better for the animal is if a weaker one that would lose a fight can scare away a stronger one. As a result, animals have a number of ways to demonstrate strength, or at least look dangerous even if they aren’t. Loud noises and postures meant to look taller or larger demonstrate size. Animals might puff up to similar effect, or extend hair or appendages to similar effect. Showing off weapons, such as extending claws or showing teeth, is done in some. Poisonous animals often have certain colors and patterns, some nonpoisonous ones copy these. Similar displays might be used to show an animal will fight hard to defend itself, and/or be more trouble then it is worth to attack. Small animals getting noisy and aggressive when defending offspring is a stereotypical example.
Similarly, if an animal would lose a fight, avoiding it is better. Camouflage, hiding, or simply avoiding bigger or dangerous animals or running away is common behavior. Defenses often serve the role of making an animal too difficult to attack to be worth it. As an example, shelled animals like snails or turtles withdrawn into a shell obviously can’t fight back, but injuring them or waiting them out would take enough extra time and effort that most attackers don’t bother. Built shelters like beehives, termite mounds, burrows, or wooden nests or dams can be thought of as protection or hiding, they are an extra layer for attackers to get through and can help the animals in them avoid being spotted, if the potential attacker doesn’t recognize the structure for what it is.
If a fight or attack does happen, animals have a number of common ways of attacking. Raw physical strength can be used to hold or squeeze the target, such as worms wrapping around each other or constrictor snakes squeezing, or squid tentacles holding prey in place. Hard body parts make attacks more effective, they can cut, smash, puncture, or scrape as with teeth or claws. Poisons can be stored ahead of time and require a lot less physical force to use, though the animals needs a way to stay safe from its own poisons, the chemicals take energy to make, and offensive use of poison needs some way to get it into the target.
Attacks can be delivered using the whole body as in ramming or constricting. Mouths are already used to eat and are already used to chew or attack prey, so putting weapons on them makes a lot of sense, as in biting teeth or venomous bites. Appendages such as arms or tentacles are flexible and extend an animal’s reach, weapons such as jellyfish stings or claws often go on the ends of these.
The obvious physical defense is armor, hard body parts such as shells, bones, or thick skin stop cuts and stings and can protect against blunt attacks. Thick padding of some sort, such as fat or less important organs, does similar, absorbing hits and stopping more important body parts from being damaged. Chemicals can be used defensively to cause pain, poison, bad smells or taste, or other effects. Poisons used defensively are sprayed in a few animals, but mostly are found inside the animals body (if it bites you and you get poisoned, its venom, if you bite it and get poisoned, it’s a poison. In technical speak anyway. General word use blurs these, with venom kept as something injected by bite.) Fur or spines or similar luff can act as a defense, by making targeting more difficult.
When a predator attacks prey, the point is to kill the prey. Thus the predator won’t demonstrate or intimidate to scare the prey, instead going right for a kill. This could be an ambush, where the predator attacks without being spotted. Attacking from a hiding spot in vegetation or rough terrain is the obvious way to do this, but predators could also attack from behind, below or above in 3d environments, or from a different medium like a burrow or into or out of water, as antlions and crocodiles do. Traps can be used, as with the stereotypical spider web. Some predators openly attack prey, either chasing it down or just positioning themselves where prey can’t do much.
Prey defending, or animals facing attacks for other reasons, will mostly try to escape or run away. There’s usually little benefit to trying to fight a predator, which is often a more dangerous animal, plus the chance to hurt, maybe kill a single predator out of lots won’t improve safety much. If the prey does stick around, such as being cornered or protecting offspring or members of a group or territory, they will fight very hard as they have a lot to lose, and likely use as many intimidation techniques as possible to scare off the attacker.
Fights over resources can more commonly be less intense, though with lots of variation. In many cases they are just animals displaying to each other until one of them backs off. The lack of immediate life or death results of a fight lead to these outcomes, though animals will fight harder for limited territory or food. If animals within a species fight each other, such as a Mating Season fight, they can often seem ritualistic, with a certain sequence of events followed. Of course, notice the qualifiers, less intense fighting is a trend, not certain, and in some animals fights for mates or territory or such get quite dangerous.
Senses are anything used to gather information from the outside world. You may have heard of human senses like vision, hearing, proprioception, and others, other animals have these as well, plus some weird ones. Here’s roughly how different senses work:
- Sight: Information from visible light, infrared, near ultraviolet. Visible light and nearby electromagnetic radiation on the spectrum have a number of useful qualities for animal senses. It can see fine details,radiation like microwaves or radio lacks fine enough resolution to do this. The sun emits more visible light than other parts of the spectrum, and Earth’s atmosphere is trans[arent (mostly) to it, so there is a lot available to see with. It interacts with many molecules, so will provide useful information, but doesn’t destroy/damage biological molecules as higher energy electromagnetic waves do, so it can be safely admitted into a sensitive sense organ.
Light sensors range in complexity from a simple light/dark sensor which can tell shadows and night/day cycles, to full on eyes that can see several colors and fine detail. All eyes have to have a receptor, usually this is a molecule that absorbs some range of frequencies. When light is absorbed, it enters something called an excited state, this excited state then reacts with something in a neuron, sending a neural signal. Eyes that see a picture need some way of resolving images. Vertebrate and Cephalopod eyes use a single lens and sheet of light receptors, the retina, in back to do this. The lens focuses the image, the array of receptor cells processes different parts. Snails have pinhole eyes, where a small hole focuses the image somewhat.
Light comes in a range of frequencies, and different materials reflect or emit a different combination of these. Color vision is how animals detect these differences. If the animal has one type of receptor that sees brightness, it is equivalent to black and white vision. In animals with multiple color receptors, the different receptors respond strongly or weakly or other amounts to different frequencies of light hitting them, the animal then combines this information to make color. Humans have three receptors, one reacts most strongly to blue and gets weaker away from this color, one the same with green, and one the same with yellow. Red light activates this last receptor but not the first two, so emitting different combinations of red, blue, and green light is a good way to simulate all colors for people. Other animals, such as birds, have four color vision. Red-Green Color blind people and many mammals have two color vision. More color receptors can see a wider array of colors, but capture less total light, at less detail, than the same number of fewer colored receptors.
Heat vision is actually a form of sight, all bodies radiate electromagnetic waves, the exact amount depends on temperature. At earth surface temperatures, the most radiated are infrared, so infrared based vision roughly sees temperature. Some animals see near ultraviolet, the least dangerous frequencies closest to visible. Both of these use the same sorts of receptors, with the same considerations, as visible light, though heat vision needs some protection from the animal’s own emissions from body heat.
Sight is more useful, obviously, in clear materials in daytime, to land or upper water bodies. Dawn/dusk and deep water animals might either lose sight if it not useful, or get very large, very optimized eyes to catch what little light is available, the information is still useful. Underground has no light at all, mostly, though animals may still use sight for activity on the surface. - Sound and other vibration: Sound vibrating pressure waves, so to hear animals need something that can vibrate, attached to a nerve that activates when stretched and relaxed. Receptors normally react most strongly to a particular vibration frequency, animals will have lots of these to cover a wide hearing range.
Any vibration travels more easily between similar materials. Your body, and most other animals, are mostly made of water, so the material characteristics are similar between water and body material. As a result, ears in water pick up vibrations very easily. Ai has a much lower density and is more compressible, the important properties for sound travel, so land animals need a way to transmit sound from air to body material. Vibrating membranes connected to the sensory organs in some way are a common build. Body organs in general can pick up movement, and can detect vibrations, such as you feeling the thump of a loud bass. However, these are not tuned to particular sound frequencies, or as sensitive as specialized ears.
Sound is useful in all environments, moreso in water as sound travels faster and farther in it. It is hard to block completely, so provides information even in cluttered environments. - Chemical Senses: Smell and Taste. Receptors have a molecule shaped in a way that interacts with very specific types of other molecules. When such an interaction occurs, the sensor molecule changes in some way, interacts with a nerve, and sends a signal. Scientists used to think shape was the determiner, a correctly shaped molecule fits into the receptor and sets it off. Now, other mechanisms are proposed as well. Animals have a wide range of sensors, allowing smell and taste to detect a wide variety of molecules.
A mouth and nose, or equivalent, are good places for these senses if an animal has these intake organs. Air or water from the environment constantly flows past when breathing, allowing chemicals to be picked up. Food entering provides a chance to sample the food, providing more information and sensing what is in the food. - Body Position, Movement, Balance: Proprioception is the fancy word for sensing where all the parts of a body are in relation to each other.
Several senses together detect these things. Sensors inside a body detect pressure and muscle contraction, together these can determine how body parts are positioned in relation to each other. Balance and movement often use fluid filled capsules. The fluid moves in response to animal movements, sensors detect this movement. Gravity can be detected by sensing pressure and force over the body, gravity pulls downward constantly across all mass, and cells can measure this. This is contrasted with wind and touch and other forces, that affect one portion of a body much more than others. Some also use fluid capsules to capture it, fluid near the bottom is at a higher pressure. - Pressure, touch, temperature, and other contact senses: These senses also help detect sound and movement, in addition to the familiar senses in skin. The exact method that force sensors detect what they do isn’t known, it may be that distortion of the cell changes electrical properties and sets the signal off. Temperature is detected with chemicals that change at different temperatures.
You might hear that “hot and cold senses heat flow, not temperature’’: This isn’t strictly true, but there is a reason for the saying: Since skin is a boundary between the body and the environment, it takes a temperature between them. The exact temperature depends on heat transfer rate. - Pain: Pain detects tissue damage. Animal pain is sensed when certain cells are damaged, sending a strong signal.
- Internal body senses: Hunger, thirst, heat, and other senses usually use the external ones to get information with the body. A cavity or organ expanding is detectable by pressure sensor, as with fullness or needing to pee. Chemical sensors can detect lack of nutrients by the concentration of chemicals. Other sensors can detect body states in similar ways.
- Exotic senses: a few animals have unusual senses beyond these. Most famous are some birds that seem to have a magnetic sense.
- Direction, sensitivity location: Humans have two eyes pointing forward and two ears fixed on either side of the head. Snails have two eye stalks looking any direction and weak ear equivalents that detect vibrations but don’t tell them apart well, Horses have moveable ears and eyes on both sides of their head. Andstarfish have lots of distributed sensors to detect light, chemicals, vibrations, and physical motion, though chemical sensors are much better than the others.
Sensory organs take nutrients and energy to grow and function, the extra information gained must be worth this cost. The same goes for extra sensory organs. Multiple organs of the same type don’t add as much as the first one, but can add coverage of different parts of the body, or if covering overlapping areas add a directional or distance sense, if angles of strength of responses are different between the two organs. Sound and light, traveling mostly in straight lines, benefit a lot from this. A body blocks light and interferes with sound, so organs in multiple directions can detect more. Organs pointed the same way provide distance and direction, as straight line travel makes sounds and light measurably different even between closely spaced sensors (Like the two slightly different images in your eyes) Smells tend to permeate an area, so multiple organs are less useful, though some animals get use out of directional smelling. Pain, pressure, touch, and similar only detect things in the sensor’s immediate area, so a body must be filled with these everywhere to get useful information.
Sense organs can have additional muscles to move them around. This allows looking at more directions with a smaller organ, but the muscles and support needed must be grown and take energy. Animals might also move entire body parts to look/hear/smell around, which removes a need for specialized muscles but does require energy and means the attached body part might have a bad orientation for some other role.
All around sensing is useful if an animal must detect things from all directions, such as predators. Thus our horses and other big hooved animals have 2 eyes on both sides of their head to see all directions, and ears facing both directions that can move. Sensors facing the same direction are useful if an animal needs to judge distances or directions well, such as primates moving through trees or predators judging an attack. Sense arrangement might be mixed, since both requirements are at work in an animal's life. I sense won;t exist if an animal doesn’t get enough information from it to be worth the extra organ. Blind burrowing animals are an example.
For senses to be useful, animals have to do something with this information. Which leads to:
At one end, we have the Clever Octopus, elephants that never forget, playful dolphin, and Clever Crows. On the other end, sponges and other attached ocean filter feeders don’t think much or at all. In the middle, everyone reading this page. Intelligence is difficult to define exactly: memory, problem solving skills, tool use, learning, adaptability to new situations all play a role, and different animals considered intelligent are good at different things. Scientists can test intelligence by seeing how animals perform various tasks, such as the mouse in a maze, or escape or tool use tests.
Any multicellular life needs a way for different parts to coordinate. Even in the simplest types, cells that move together or grow together will do better than ones that don’t. If the life form has senses, this information has to be processed in some wary to be useful. To accomplish this, various communication methods are used, which cells respond to. Chemical communication is the most basic, using various signaling molecules for this role, which diffuse or are pumped to appropriate receptors and cells with appropriate receptors to bind these molecules and respond appropriately.. Hormones are an example, in humans and many other animals.
More precise and faster, but less long lasting, are electrical signals. These take the form of nerves most of the time. Neurons, the cell involved, have a long main tube with several branches on both ends. Signals are transmitted by ion exchange along the tube, when the end is reached the neuron releases chemicals to another connected neuron or to another type of cell that does something, these chemicals set off another electrical signal. Neurons can deliver a signal to exactly where it is needed, instead of diffusing like chemical signals, but the signal disappears quickly. The chemicals that communicate between neurons, called neurotransmitters, can act like chemical signals/hormones, lasting longer and diffusing over a wider area. Learning and memory are thought to come from neural connections being strengthened or weakened, but the nervous system is complex and other processes may play a role.
Chemical signals in theory could produce what we call intelligence, but in practice neurons take this role. Neural webs in animals like jellyfish make decent decisions, but as animals get more complex, lumps of neurons are used to fill this role. In animals with heads, the brain is the biggest lump, sometimes a few additional ones exist in the body. A measure called “encephalization quotient" compares an animal’s brain size to an “expected” size for the animal’s weight, which is usually calculated by looking at lots of similar animals, checking ratios, and calculating a trend. This measure roughly corresponds to measurements of intelligence, but not in an exact way. Larger bodies need larger brains to process basic sensory and movement data, otherwise more neurons allows more types of complex networks and connections and should allow more of all kinds of intelligence, but some animals may use what they do have more efficiently.
Neural tissue eats up a lot of energy, in your own body about a quarter of the energy you use goes to your brain, so intelligence has to have a good payoff, situations requiring memory or adaptability tend to have more intelligent animals. Oceanic nonmoving filter feeders don’t move or make much of any decisions at all, so intelligence is low. Social Animals are often more intelligent than solitary ones, as keeping track of members of their group and navigating relationships can get complicated. Animals in complex environments use intelligence to navigate them. Carnivores are thought to generally be more intelligent than herbivores, since their food sources can move and have to be tracked or searched for. Animals with more generalized diets are likely more intelligent, this allows them to adapt to new food sources and handle a greater variety.
Almost all animals interact with members of their own species, even if only to mate or they happen live near each other and interact that way. Some take this further, living in groups of varying complexity, ranging from temporary groups coming together to do something like hunt, mate, or scare off predators, to long lasting groups like herds, to hive insects with inborn specialized roles and huge numbers of members. Wherever animals contact each other, some sort of social interaction and communication occurs.
Grouping up, like anything else animals do, has tradeoffs. Disadvantages include that more animals in a single space means more competition for resources. Animals must also manage interactions between each other. Advantages of grouping up are that members can work together in ways that benefit everyone in the group, such as working together to get resources they could not individually, like building things or hunting large prey if carnivores, Animals can specialize, some taking different roles like different hunting positions, or some animals watching for danger while others do something else.
The combination of competition for resources and cooperation can make social interaction rather complicated, so more social animals tend to be more intelligent than equivalents. A number of structures can exist, such as highly specialized eusocial animals where every individual is specialized to do something, to family groups, among others. Common is hierarchy, where higher ranking members get more resources than lower ranking ones, animals will compete for positions in the hierarchy at points but otherwise accept a position. This can reduce fighting in some situations compared to all animals competing all the time for resources.
For animals to interact, they must communicate. Communication can use any external sense an animal has, with one sending a signal to trigger that sense and the other detecting the signal and acting on it. Calls, songs, and other sounds are most familiar, sound is great for communicating as it travels around obstacles, goes a long distance, and has a lot of potential variety allowing lots of potential messages to be quickly sent. Vision may be used with body position and movement, or in a few animals light or color displays, Chemicals used for smell communication are usually called pheromones, these can last a long time in an area but are not as fast as sound or light. Touch can be used as well, as in rubbing someone to comfort them.
Cross species social interaction is somewhat common. Most well known to readers are human and domesticated animals, but different nonhuman animals will sometimes join together. Examples are some multispecies herds, or occasioanlly different types of animals teaming up to hunt. Animals must be able to follow each other's communication, which will make this process less common, but combining different capabilities, or different requirements, is beneficial. Multispecies herds, for example, might have some species as better lookouts and some as better fighters, and different food requirements reduce competition.
Reproduction is one of the defining features of life, animals are no exception. Like other multicellular life, animals reproduce by creating a smaller, separate body that grows into a full grown organism. A few can reproduce asexually, one animal producing offspring on its own, almost all reproduce sexually, combining genes from two parents in each offspring. Thus an entire collection of Sex Tropes. Sexual reproduction allows more mixing of genetic material, creating more variety in offspring, though the two sets of genetic material need some way to meet, which involves risk and energy.
Sexually reproducing animals start from a single cell, created by merging a cell from each parent. The merged cell divides to form a mass of cells, this cluster grows large enough to form a small body, portions of which specialize into different organs. Chemical signals control how this specialization goes, based on the location of cells within the structure, ultimately the whole process is controlled genetically in some nonobvious ways, there are often not specific genes for, say, a tentacle or lung, but combinations that produce a particular pattern. Mutations in these genes are a common way to evolve unusual new body types. At some point, the body is large enough and has the correct organs to live as an independent organism.
In the simplest form of reproduction, reproductive cells are released by parents, randomly meet, carry enough starting nutrients to form a small, independent offspring, and that offspring than goes on to live with no further assistance from the parents. This type of reproduction has the advantage of the parents needing little energy per reproductive cell, they can produce a lot, some of which will survive. Over time, reproductive cells have specialized, some are larger and carry nutrients to start growth, these are eggs. Others are smaller and cal usually move faster, these are sperm. In animals that specialize in producing one or the other, egg producers are females, sperm producers are males, you are probably aware of other differences that have come from this division in your favorite animals of choice. Producing both is an option evolved in some animals, it requires these animals to maintain organs to produce both but adds flexibility.
Some animals invest a bit more energy into each offspring, these can’t throw out as many reproductive cells, but the ones they do release have a better chance of surviving. Eggs can carry more nutrients, often the egg cell is in a larger structures such as the chicken eggs we eat, that carries lots of starter nutrients for a good sized offspring. Parents can guard eggs, maybe burying them, maybe watching them or carrying the eggs, or dropping the egg structure and growing offspring inside their own body. When the offspring hatches or is born, parents may further protect and take care of it for some time, thus many Parenting Tropes.\\
Simply spitting out reproductive cells on the chance some will meet works for some animals, but most have to meet up close in some way to bring sperm and eggs together, a.k.a. mating (...Sex Tropes). This also allows animals to choose who to reproduce with, though in some sparsely populated areas animals will go with whoever they meet first as otherwise they might not reproduce at all, At minimum, the animals can stay near each other which makes sure sperm and eggs are close as well and can meet, animals can do internal fertilization where one type of cell (usually the sperm) goes into the other’s body, using a range of different methods in different animals. In between options exist, where sperm is given to the receiver, who puts it on their own eggs or into their own body. These methods are important for land animals, without water reproductive cells can’t move freely to meet.
The choice of who to mate with has goes a number of different ways. Animals often go for qualities with survival advantages, such as faster or stronger partners. Some animals fight for the chance to reproduce, or use competitive displays in a similar role. A lot of animals go for qualities without obvious survival benefits, such as birds liking nice songs or colorful display structures, a combination of animals with the characteristic and animals attracted to it can cause runaway evolution where the feature takes over a population or forms a new one. Meeting potential mates is easier if everyone is close together and reproducing at the same time, this the Mating Season Madness trope in many animals if their way of living allows it. Animals may have a few to many partners, and spend varying amounts of time with them, from attached filter feeders that release reproductive cells and never meet reproductive partners, to one off encounters, to herds where one dominant animals reproduces with many others from the opposite sex, to longer and shorter term monogamous pairings.
Two animals have reproduced, a new creature is hatched, born, or otherwise come into existence. Now it has to grow to adult size. Growth rates vary from animal to animal, as a general trend, animals with faster metabolism will likely grow faster. Food availability is also important, the more food, the more an animal can eat, the more it can grow. Still, there is wide variation. Animals with parental care will be fed or otherwise taken care of for part of this time, animals in social groups are often cared for by members of the group.
Some animals are born like miniature adults, some go through changes. The most extreme of these changes are a process called metamorphosis, where the animal goes from a larval form to a completely different body shape. Caterpillar to butterfly is the most well known example, other insects, some bony fish, starfish, amd a number of more obscure animals do something similar, including a number of attached, sessile animals that have swimming larvae before changing to settle down. In between are creatures like frogs with tadpoles or young wingless dragonflies, which noticeably change but still kee[ big elements of their bodies from young to adult forms. Being hatched or born adult like obviously requires less change over time, saving some energy and requiring less genetics or signaling for development. However, changing over time allows different forms to specialize into different roles, or do things when young not necessary for adults. Insect larvae often specialize to eat and don’t use energy on reproductive organs, sessile nimal’s larvae can swim to find new locations, before attaching somewhere as an adult. Among many animals, metamorphosing ornot, adults and young eat different foods and possibly live in different places, reducing competition with themselves.
At the other end of the lifecycle, animals face a lot of deadly threats, plus aging in many. Lots of deaths are of course caused by diseases, attacks, accidents, extreme weather or lack of food or other bad luck, and other such things, animals in human controlled environments can almost always outlive animals in the wild. However, some internal controls do seem to limit how long animals live even if nothing external kills them, though mechanisms of aging aren’t well understood. Some animals die after reproducing, cephalopods and some insects are the most famous, the tradeoff here is likely that putting energy into reproduction instead of surviving gives more offspring, which compensates for the original parents dying.
Disease and injuries are constant threats, they can kill directly, or weaken an animal to where it dies some other way. Animals have a number of ways to survive and heal from them. Protective systems like skin or acidic or alkaline stomach stop many microbes and parasites from entering in the first place, usually either with barriers that physically stop an infector from getting in, or chemicals that poison or destroy it. Large enough parasites can be physically removed, as with horse or cow tails that can swat flies, or birds and whales rubbing themselves in dirt to clean off skin parasites. Some animals specialize in cleaning parasites from other species, usually behavior evolves so the animal being cleaned allows this to happen.
If microbes get into a body, animals have further defenses. Some chemicals inside a body can harm microbes. Specialized cells can eat them, other cells can attack infected existing cells, which are part of a virus reproductive cycle. Vertebrate have additional components to this system, that can adapt to specific invaders and more effectively target them, cleaning up the disease agent much faster if exposed again. This is the basis for vaccines, which expose the body to something harmless that activates the immune system as a disease would, so you catch the actual disease your body fends it off far faster. These disease fighting systems also attack cancer cells, some smaller parasites, and other random things that may enter the body. To tell an outside body from a part of the animal, animal cells have signaling molecules on their surfaces that invading disease microbes usually lack, immune cells can also respond to common molecules found on disease causing microbes to detect them.
To heal from injuries, an animal needs to remove damaged tissue if it can’t be repaired, and regrow whatever was missing. Doing this takes nutrients and energy, and some body parts on some animals can’t be fully healed if damaged. To remove unrecoverable tissue, some will be rubbed off, sometimes regrowing tissue will push it away, immune cells might digest some if the tissue comes in small enough pieces. Regrowing tissues means lots of cell division, with chemical signals to tell what type of cells should grow where, and chemicals such as bone or shell are secreted as needed. The process is about the same as normal growth, since regrowing what is damaged is what the animal is doing.
If an animal isn’t killed by an attack, accident, or infection, or other bad events, a few patterns have been found for how long they can live. Larger species tend to live longer, though smaller animals within a species tend to live longer also. Animals that face fewer threats/have some protection, like flying or shelled ones, tend to live longer, likely the benefits of reduced aging matter less when something else has a good chance of killing it anyway. Slower metabolism animals can live longer, though this isn’t as consistent a trend, it seems the lowered speed of chemical reactions means those associated with aging also happen slower. Some cnidarians are unique in switching between two forms, each of which effectively rebirths the animals, these theoretically can live forever this way. Some others don’t seem to age at all, though they still suffer other causes of death. The longest lived animals as far as we know are some several thousand year old sponges and corals, out of larger animals some deep sea sharks may be the longest lived.
Horses, Cows, Dogs, and many more: Human society uses a lot of animals, mostly for food, work, and companionship, also entertainment like cockfights or horse races, and various other random small roles. Animals might be wild, as with hunted ones or pollinators, that happen to serve a role for people. They may be tamed or kept in captivity, still the same as a wild animal but controlled or learned to live around humans, as with aquarium fish, honeybees, or War Elephants. The most significant involvement is domestication, where an animal has changed in big ways from a wild form into something adapted to live with humans. Wolves to dogs, most livestock and horses, are all changed in this way. Some animals cannot be domesticated for reasons that aren’t entirely clear. Even if not domesticated, animals for taming should be useable if they are comfortable around people or can be controlled appropriately.
Food animals might be eaten directly as meat, or kept for products such as milk, eggs, or honey. Good food animals historically ate foods that people wouldn’t want, such as waste fed to pigs or grass eating livestock: if the animal diet did overlap, plant food was needed. This way the animals weren’t competing with people for food. Though in modern times with extremely productive farming, food animals mostly get a much richer diet of food crops. Animal products supply nutrition that is hard to get elsewhere, meat milk and eggs give people fat, protein, and a number of vitamins that are possible but trickier to get from plants. Fish historically have been caught wild, but in the past few decades controlled fish farming has gotten more important, especially as fisheries in many places have become less productive thanks to overfishing and pollution. Fast growth is useful, for meat animals this often means less food needed per meat produced. It also often means less food for the animal is needed to produce whatever it is being raised for, an important quality for anything being grown.
Work animals like working dogs or draft horses obviously need to be able to do the work expected: draft animals need to be strong and have good endurance, hunting dogs need their good smell and hearing, plus endurance for a longer hunt, you’ll know what qualities such an animal needs for the work they do. Just like with food animals, it is good for work animals to eat something different than human food, so as to not compete directly, however the roles work animals are playing may be valuable enough that this isn’t a concern. Among work animals are war animals: Horses, War Elephants are famous, camels have been used to great effect in some battles. In battles before industrialization, these animals give their riders height and speed, and large animals coming to kill you, often making powerful noise, are very scary; getting enemies to run away is a major part of these battles.
As for pets, entertainment, etc., go for whatever makes you happy and isn’t too expensive or causes too much trouble.Using animals for human society does bring a lot of problems. Animals moved around the world can become invasive species, outcompeting native animals and causing problems in nearby environments. Rats, dogs, and cats are known to do this numerous places in the world, pythons in Florida are a more exotic example. Animals can be dangerous, large livestock kill a good number of people each year. Animals pollute, in premodern times dung was the main problem to clean up, today the land use that often replaces forests or other denser plant cover with animals farming is the big problem, contributing to global warming. Beef cows are notorious for this, also giving off methane that adds to the problem, plus the pollution associated with feed often used for them. Human used animals are often densely kept, spreading diseases. On a smaller scale, catching exotic pets often uses highly destructive practices for the amount captured.
Still, despite these issues, animals will be used for a long time.
Types of Animals
Since fossils of most types of animals are uncommon, scientists have used molecular phylogenetics, comparing mutations and mutation rates of different organisms, to figure out what is related. This useful notes mostly describes modern animals, for detail on prehistoric animals, check out Prehistoric Life or The Other Wiki or any other books in question, though you will get some details here.
The animal family tree has several groups, sponges, then comb jellies, then cnidarians split from everything else, until bilateral animals are reached. Bilaterals split into protostomes and deuterostomes, protostomes split into ecdysozoa and spiralia, and these groups have further family trees that won’t be described here.
Avoiding Small Taxonomy Pools is going to be tricky, there are about a million known animal species, and some estimates go to 5 million or tens of millions, mostly insects, defaulting to the best known ones is understandable. But possibly these groups give you some ideas.
Non-Bilaterians in general
The “primitive” animals, either with no symmetry or radial/circular symmetry. They have lots of unusual features and abilities you don’t find among bilaterians.
Non-moving filter feeders, these are probably the closest to the earliest animals. Sponges have a similar form, a water channel or series of channels on the inside, lined by a layer of tissue, that coats a layer of structural material of some sort, followed by an outer lining. The inner layer is lined by cells with cilia, the cilia pull water through the inner channel, where particles and microbes are absorbed as food. More complex sponges have lots of small channels, giving more surface area for the cilia to pump water and absorb nutrients. Sponges use a number of structural materials for support, some use a material called spongin similar to collagen, some use minerals.
The cilia containing cells resemble animal’s closest single celled relatives, possibly these formed a colony and created the first animals. Most likely early animals were similar to sponges in being attached filter feeders, so sponges are likely the leftovers of these earl creatures.
And if you are wondering if sponges the animal were used for sponges the cleaning tool, yes, they were. Spongin is elastic, and the numerous water channels and holes in some sponges are excellent for holding water, combine them and you get the soft, elastic, water soaking thing found in kitchens and cleaning equipment everywhere. Sponges were used so much for this that they were overfished in some areas, synthetic materials got rid of this problem and are the main component of cleaning sponges today.
The transparent cilia carriers on comb jellies can have a number of shapes, lobes, ovals, twisted, or squash shapes.
Coral, Sea Anemones, Jellyfish, and some others. Cnidarians have an unusual reproduction system, cycling between a fixed in place form and a free floating or swimming form; a few species can in theory live forever by changing between these forms. Cnidarians are radially symmetric, the same going around in a circle, and have a digestive system where food goes in and waste goes out through the same opening, many have some sort of tentacles or arms utilized in capturing prey. Cnidarians are also characterized by the possession of cnidocytes, cells that are shaped like tiny harpoons that serve to inject venom into their prey.
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Jellyfish
Tropes:
Electric Jellyfish
Used often as background animals swimming in an ocean, or documentaries as a generic ‘simple animal”. Jellyfish are not any particular branch of cnidarian, instead are found in several groups. The tentacles sting things, and draw food towards the jellyfish mouth where it is eaten. Their pulsing swimming motion takes advantage of water turbulence, helping the cup structure relax. The floating jellyfish we are familiar with is just one part of a reproductive cycle, also part is a seabed living fixed form that spawns many swimming forms.
Jellyfish stings: The sting actually injects a structure into the victim, this structure releases poison. To get rid of the poison, the best way is washing with seawater..yes, a sea creature’s sting is solved with seawater. This washes off the jellyfish structures without popping them as freshwater might.
These days, with lots of ocean creatures dying off for various reasons (pollution, temperature, acidification, overfishing) jellyfish have been filling in the gaps. Not too economically useful, cause cause further problems if jellyfish eat too much larvae of other creatures, its one of many environmental issues going today. Though many sea turtles eat the things, they too are negatively impacted by the ocean's changing climate and pollution, so may not be enough to curb the jellyfish.
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Siphonophores
Tropes:
The Worm That Walks
Also not a specific branch of cnidarian. Siphonophores form big colonies of specialized animals, in a vaguely similar way as cells form multicellular life, and live much like jellyfish, eating and often stinging what they can catch.
Among their number are some of the few really huge non-vertebrate animals. One of them is longer than a blue whale, though not as heavy and technically made of lots of animals.
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Coral
Makers of reefs, with lots of beautiful, colored shapes where tons and tons of other life makes its home. These shapes are actually rocky structures built by coral, the actual animals are small structures not too different from jellyfish or sea anemones, with tentacles surrounding a mouth. Many coral have algae within them that supply much of their energy, otherwise they catch whatever floats by.
The reefs are much more known than the animals themselves, creating fantastically productive ocean systems that produce a lot with often limited nutrients, and act as nurseries or otherwise interact with the wider ocean. Not surprisingly, these are threatened by pollution, temperature changes, and acidification, a common effect is “bleaching” where coral lose their algae, go white, and die, causing problems for everything else living nearby.
Bilaterian Animals in general
Most animals around today, including most of the familiar ones. Symmetric from side to side, with a full digestive canal, one side takes in food, one side ejects the waste, if you know what we mean. Sensory organs are concentrated towards the mouth end, the direction the animal usually moves toward, and a brain if it exists is near these sensory organs. The original was probably wormlike, modern forms have diversified into lots of types of worms, plus most of the more familiar animal groups.
Bilaterians are divided into protostomes, where the first opening in an embryo becomes a mouth, and deuterostomes, where this first opening becomes the other end. Protostomes are further divided into animals that molt (mostly), and animals with a certain type of larval development (cells form a spiral pattern as they divide.) Plus a few exceptions that branched off earlier from these groups, but these are rare.
Some animals can be rotated and look about the same, some look mirrored side to side (mostly, some internal organs may be biased toward one side, such as your spleen or stomache), some don't seem to have any particular shape at all. What gives? and why aren't other forms of symmetry common?
Scientist's best guess, as with a lot of animals evolution, is that this is based on how animals move and feed, plus how growth and development works. If an animal moves a lot, there is difference between the end facing where it is going and the end facing where it came from. Because animals are living on earth, up and down also matter. This leaves the side to side direction as not mattering much. If an animal doesn't move much, and things come to it, in most cases they could come from any direction, which means only the up/down distinction matters. Animals that filter feed, or otherwise catch drifting material and don't move quickly tend to be irregular or radially symmetric. When the first bilateral animals evolved, they were likely better suited to move quickly, and this let them diversify. But some went back to slow movement and attached filter feeding, and these again became irregular or radially symmetric. Symmetry has some uses in development, a repeating pattern can help development by repeating the same set of steps several times.
Like a lot of the world, there are lots of exceptions, like swimming jellyfish or bilateral clams, but the pattern holds pretty well. You can see a similar pattern with machinery or vehicles, things like airplanes and cars tend to be bilaterally symmetric, with the forward backward and bottom to top directions mattering.
Ecdysozoa
Moulting animals. These have an outer covering that must be shed periodically as the animal grows, after which a new covering is formed in its place. This is a dangerous time for the animal, especially if the covering provides protection as in arthropods. Many in this group came from a segmented ancestor.
Roundworms and lots of other little tiny things, including lots of parasites. You don’t notice them day to day unless you or a pet gets infected, but they are one of the most common animals in the biosphere, found in all environments, including on you. Some estimates suggest they have about a million species, but scientists have only discovered a few tens of thousands. Probably the most unusual environment is in the deepest mine in the world, where some nematodes eat bacteria living on the rock. It is thought they descend from worms introduced when people first dug the mine. Nematode diets are pretty much anything small they can get their hands on: bacteria, small particles like skin flakes, decomposing material, bits of flesh and body nutrients as parasites, etc.
As the name roundworm suggests, these are pretty simple animals, a worm that is round, with a standard gut and head, with sometimes an organ for sucking liquids. Nematode mouths are somewhere between a Flower Mouth and a regular jaw, with several components that open and close together.
Technically, nematodes are part of a bigger group, the nematoida, which includes several similar worms.
More worms, living in the ocean, eating random bits of stuff. Mainly included because of their name, meaning “Penis worm”. (Same base as “Priapism”.) This both references their general shape, and a particular organ.
The Kinorhyncha, close relatives, live in mud instead, and eat random bits of stuff there. These are more obviously segmented, but still wormlike.
Both these animals have an outer hardened cuticle, which has to be molted.
Despite the name, these look something like caterpillars, and unusually are fully land based, though must have water based ancestors as their relationship to arthropods seems to go back to when these were water living. Instead of legs, they are supported by cones of their body extending to the ground on which they wriggle. These lower portions are hardened, while the rest of the skin is softer.
They are mostly carnivores, ambushing prey, and some show herdlike behavior, defending regions and developing a dominance hierarchy.
Also called water bears. They look something like an armadillo bug if such a thing existed, with several feet attached to shelled segments bigger near the middle and smaller at the head and end. And they are very, very microscopic. Mostly they eat microbes and plants, a few eat fellow tardigrades.
Their main claim to fame is surviving all sorts of nasty stuff: high and low temperatures, drying, outer space, quantum entanglement, among others. A simple genome helps with this, the animals also are able to enter a dormant state that can tolerate these conditions and restart under better conditions. They also have a number of proteins that preserve cellular structure.
Tropes:
Big Creepy-Crawlies
In many ways, the stars of the animal show, living in almost all environments, by far the most species both discovered and estimated, filling almost all possible ways of eating and living. Prehistorically, members of this group were the first animals onto land and into the air.
Arthropods are covered in a hard exoskeleton, and form in segments, you can see these clearly in a lobster shell or on centipedes or millipedes. In most creatures the segments have evolved to merge together into a few large body sections, as in three part insects or two part spiders. This exoskeleton commonly has appendages, forming legs, arms, claws, antennas, or wings based on the creature. This skeleton provides protection and support, but for whatever reason never evolved to grow with the animal, the danger during moulting and slowed growth limit arthropod's modern size. Compound eyes are common, with lots of different lenses adding up to a single picture, but a number of eye structures exist in the group.
With such a large, well known group, it's worth writing up arthropod subgroups as their own sections.
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Chelicerates
Tropes:
Arachnid Index
Scorpions
Arachnids and horseshoe crabs, named after their mouthparts. Arachnids include spiders, scorpions, and ticks, among others. Prehistorically, these may have been the first animals onto land, legs and organs easily adapted to book lungs made the transition relatively easy. Today, horseshoe crabs still make this transition, going on on a beach to mate and lay eggs, while arachnids mostly live on land permanently.
Spiders and scorpions have merged body segments into a head and rear section, the head uses organs called pedipalps to eat, the rear section contains most internal organs. Spiderwebs are famous, but spiders can also ambush prey, or create other types of traps, sometimes using poison. Ticks are parasites, of course. Horseshoe crabs look like a dome with a spiny tail that walks along the seafloor, the tail’s main use isn’t stinging as it may appear, but flipping the crab over if it gets stuck.
Many of these animals use book lungs, a breathing system where flat, body fluid containing structures are stacked together and surrounded by air.
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Myriapods
Tropes:
Creepy Centipedes
Centipedes and millipedes. Long critters with lots and lots of legs. Centipedes are carnivores that lack eyes, millipedes are usually herbivores or random debris eaters that can see, among other differences. They are a candidate for early land animals, breathing through leg structures, today all of them live on land. Centipede bites may be painful, but rarely dangerous.
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Crustaceans
Tropes:
Giant Enemy Crab
Red Live Lobster
Technically not a phylogenetic group, since insects descend from some of these, but still recognizable as a type of animal. Often 10 legs, shelled, includes culinary favorites shrimp, crabs, and lobsters. And, surprisingly, barnacles, which resemble free floating crustaceans until they attach to something and filter feed, to the bane of large sea creatures and ships everywhere. Mostly they live in water, some go on land for short periods, and a few have gone fully land based.
Crustaceans come in many sizes, from the largest arthropod today to krill, and fill lots of ecological roles in the ocean, from detritus eaters to somewhat large predators, one the seafloor or floating or swimming, and act as a source of prey for lots of larger animals. The commonly known lobsters and crabs, and many shrimp, are in a group called decapods, meaning 10 feet, but other groups have more segments and look like worms with lumps or millipedes. Lots of animals are called shrimp, unlike the curled cooked version, most of these are flat when alive. They are not red as the cooked lobster trope suggests, see uncooked shrimp as a better representation of colors, though some crabs will look red.
Tropes:
Insect Index
By species, the biggest group of animals, more than all other animals put together by about 2-3 times at around a million. Estimated species undiscovered could be something like 5 million. Yes, bugs and bees and beetles are pretty important. They are descended from crustaceans, unfortunately their fossil record is thin so how this happened is not well known. Around 350 million years ago, ish, flight evolved in some insects, and they have been massively diversifying since that time. The extreme number of species seems a mix of small size allowing lots of specialization, fast breeding cycles allowing lots of changes, and flight, as birds and bats similarly have lots of species compared to other similar groups. Some estimates come from finding insects within species of tree, and discovering new insects at a decent clip, with new species occuring every time a new plant species is checked. Such extreme association with some other life form shows up a lot, with insects that pollinate a single species, or parasitize one particular type of animal, or burrow in a particular plant, repeat this over and over, and you get a lot of animals.
Insects follow a similar body plan. They have three main sections, a head with eyes, antennae, the biggest chunk of neurons (insects have several such clusters, the biggest is the closest to a brain in other animals), a mouth, all the usual parts, a thorax where the legs and wings attach where several internal organs hang out, and an abdomen which is usually the longest section, with more internal organs. As the name hexapod suggests, insects have three legs, running motion is usually by lifting one tripod (a front, a back on one side, a middle on opposite side) and planting it, followed by the other. Insect wings probably evolved from lost crustacean legs, which would have become lumps that could develop into full wings, though some other guesses exist. To move, the thorax near the wings is flexed and rebounds, wings can flap very, very quickly. “Physics can’t explain how insects fly” is repeated somewhat commonly: though the fluid flow is complex, the physics is understood pretty well and not out of the realm of other fluid mechanics questions. In most species, wings can be folded onto the body, taking up less space, allowing camouflage, and such. Insects breathe by letting air through tubes in their body, oxygen is absorbed and diffused into their cells. Circulation has a heart pushing body fluid through a tube, which then becomes intracellular fluid moving through the whole body until absorbed again.
If there’s a possible lifestyle that involves living on land and not being too large, some insect probably lives it. Plant eating, fruit eating, scavenging, blood sucking….forests, tundra, desert…you get the idea. They are highly important species in most ecosystems, eating, shaping the environment, and acting as a source of food for other creatures, the fact that “insectivore” is a common term should make their value as food obvious. Some lifestyles are highly unique to insects, by far the most pollinators are in this group, as are so called parasatoids, which inject larvae into a host that is gradually eaten to death as the larva grows.
Though caterpillar into butterfly is a common trope, not all insects have this sort of metamorphosis. Many of the earlier groups hatch as forms similar to the final forms, though lacking things like wings, as the animal grows and molts its body shape changes into the final form. The larvae of these types are called nymphs or naiads usually, while the full metamorphosing ones are just called larvae unless they have a special name (like maggots or caterpillars). Many insects actually spend most of their life in these larval forms, eating and growing until they change to the flying form for a quick burst of reproduction.
Most groups of insects have “opter” or “optera” or such in the name, based on the greek word for wing. (Compare helicopter or pterosaur) And now, the different types:
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Early Hexapods and non flying insects
Springtails, silverfish, and a number of other random bug looking things. They have a similar body shape apart from lacking wings, though some have mouthparts inside the body. Most of these live in forests among random stuff on the ground, eating various things there, silverfish sometimes hang out in the roughly similar environment of walls and furniture. They can be a sign of the environmental health of such areas, and walking out of old furniture can be a surprise, mostly they aren’t common in media or well known.
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Paleoptera, or dragonflies and mayflies
Tropes:
We Are as Mayflies
Dreadful Dragonfly
In prehistoric life shows, carboniferous giant dragonflies are a common sight. With no larger animals (a.k.a. Vertebrate fliers) in the air, they could get much bigger than current flying insects. In modern times, dragonflies still catch things in the air using those well known long wings. Mayflies are known for not living that long, once their larval forms turn into adult forms, they mate and die very, very quickly.
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Zoroptera, or angel insects
Soft bodies, with a particular antenna shape. One of many groups of insects to live in leaf litter or under logs. Not too many species.
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Dermaptera, the Earwigs
Flattened insets, letting them fit in small spaces. In the wild, live under logs or leaves or such, eating random bits of plants. You may have seen them crawling out of furniture, where bits of food and the medium tight fit of cushions and furniture parts mimics their preferred environment.
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Plecoptera, the Stoneflies
A less concentrated version of We Are as Mayflies applies to this group, most of their life is spent as larvae in water, eating random bits of plants or very small animals in lakes and streams, and only a few weeks in the adult flying form.
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Orthoptera, or Grasshoppers, locusts, crickets, and related
Tropes:
The Swarm
Musical Cricket
If you want an alternative to Strong Ants, Grasshoppers jumping really high is for you. Like the ant trope, it is an example of the square cube law, muscle strength increases by cross section, or the square of the size, the mass to jump increases as the sube, smaller animals can jump further compared to their size than similarly equipped large animals. Locusts are of course known for swarming and eating everything in an area, as in one of the plagues of Egypt, or a general expression. Crickets, named Jiminy of course, are the noisemakers of the group, chirping away using their legs.
All of these animals have a similar shape, long bodies, long back legs. They mainly eat grasses, sometimes other plants, as the habits of locusts suggest. Depending where you live, they may be offered as food, The Bible has locusts as an allowable thing to eat. The cricket chirp is mainly a mating call, sounds like this are used by other members but not as famously.
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Mantises
Tropes:
Mantis Mating Meal
Slaying Mantis
The praying mantis is probably the name you think of, Oversized Mantises are also common fantasy creatures, or at least some sort of giant insect borrowing the name, most likely based on a martial arts association.
Most mantises stand angled upward towards the head, with long arm looking front legs, these are used to catch prey. Though they are known for a trope named after another arthropod, this behavior isn’t a guaranteed thing, instead something that happens part of the time in some species, common enough to be noticed but not most of the time.
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Roaches and Termites, or Blattodea
Tropes:
Termite Problem
Creepy Cockroach
Hive Caste System
You’ve probably heard of the memetically hard to kill cockroach, or the wood devouring termite. Roaches tend to be omnivores, not too specialized, flat, and nocturnal, which makes the house pests good at getting into buildings, not being seen, and eating all sorts of random food or other items that you probably don’t want eaten. A lot of termites do in fact eat wood, though a lot also eat random debris. Bacteria help them digest this wood, and methane release from these bacteria is a big source of this gas in the atmosphere.
Most of these insects are quite social, roaches have lots of communication and group interaction, while termites are a famous example of an insect hive system. Termites have both a reproducing male and female, instead of a queen with smaller drones as found in other hive insects. Some termite nests can get quite large, building up into several foot or several meter tall mounds, influencing the surrounding area.
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Stick Insects, or phasmatodea
Insects shaped like sticks. Or maybe leaves. Plant eaters, with a famously camouflaged body resembling various plant parts. Some can get quite long, though their thin body shape means they aren’t as heavy as some more compact insects.
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Webspinners, or Embioptera
Spiders aren’t the only things that make silk, nor silkworms. These insects live in clusters under the webs found in their name, and mostly eat random plants or other debris. The Mantis Mating Meal could be named after them, as males once in adult form go to a different cluster, mate, and die, sometimes being eaten afterwards.
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Ice bugs and gladiators, or notoptera
This group has a small number of species, found in very narrow regions of the world. Ice bugs, as you might guess, live in very cold areas, and are so adapted they are killed if the temperature gets a little bit above freezing. They mostly eat plants and debris. The gladiators are recently discovered, though new insect species discoveries are a very normal thing, this was a rare time a completely new line of insects had been found. The gladiators are carnivores, something like mantises, found in various spots in southern Africa.
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Lice
Tropes:
Lice Episode
Not just the bloodsucking hair living bugs you want to get rid of, some also eat books, or stored food. And of course, a lot live out in the wild, eating bits of plants or other material as many other insects do. Some of them are among the few live birthing insects, and a few produce silk. Lice that infest animals get very specialized for the animal they attach to, species exist that live on most kinds of birds and mammals.
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Thrips, or Thysanoptera
What lice are to mammals and birds, thrips are to plants. Some eat debris or small prey, some pierce into plants and suck up fluids, some eat pollen or other plant material on the surface, among these are many crop pests. Many are social, the groups living in growths they cause on plants. They are good at resisting insecticides and avoiding predators that might control other pests, making them a bigger challenge to deal with.
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True Bugs, or Hemiptera
Tropes:
Cicadian Rhythym
Not those fake, pretend bugs, these are the real thing. (Scientists like to name groups “true somethings”, but you often see it in greek as the eu- prefix, as in “eusocial”) In this group you may have heard of cicadas, bedbugs, and aphids.
As you might guess from the list, these bugs have quite a variety of lifestyles. Bedbugs and a few others suck blood. Aphids feed on plant sap, others food on plants in general. Some are predators, such as the assassin bug, some of these pierce their prey and such out body fluids, or inject enzymes to digest the target. You may notice how many of these involve piercing something and sucking something out, a key feature of this group is a piercing beak that both injects and extracts liquid, with a separate channel for both roles.
The cicada noise comes from a vibrating structure that is popped in and out.
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Ants, Bees, Wasps. AndZoidberg and sawflies. or Hymenoptera
Tropes:
Bees
Bee People
Insect Gender-Bender
Insect Queen
Hive Caste System
Ant War
Strong Ants
Wicked Wasps
Probably the insect group that shows up the most in media. Though termites and social wasps (think hornets and yellowjackets) exist, most hive insect/hive society tropes are based on bees and ants, there’s just something darn impressive about insects forming such organized societies, even more impressive when you remember no one is actually in charge despite the Hive Queen trope, the whole colony is self directing, relying mainly in pheromones and some simple tricks for organization. Plus swarms do make an impression, which these insects are prone to do.
Sawflies, often forgotten, are the original members of the group. Most live the typical life of generally plant eating insects, but at some point the ability to inject larvae into other organisms involved. This created a new type of organism, the parasatoid, which injects larvae into a living host. The larvae than eats the host from the inside until fully grown, when they emerge and the host dies. (Parasatoid is the name because it isn’t exact parasitic, the host is guaranteed to die, but unlike predators the host isn’t killed right away. This won’t happen to you, they go for roughly similar sized targets as themselves, insect larvae that eat gigantic you are just regular parasites.) A nasty way to go, but since most of the targets are other insects, and most such organisms specialize in a single type of organism, these parasatoids can be useful as highly directed pest control. Wasps evolved from insects with this way of living, in many including bees the organ that injects the eggs changed into a stinger, thus creating wasp and bee stings and all their fun. Wasps have diversified into other roles, among them are ants where flight was lost in most members.
Members of this group fill most possible lifestyles, some being herbivores, some predators, some eating random debris, the mentioned parasatoids, regular parasites, etc. Bees are well known for pollination, grabbing nectar from a plant, picking up pollen, and taking it to the next flower. Using honeybees for this gives pollination, noney, and was together, making a rather useful creature to raise. (Though there is some concern about honeybees replacing more specialized pollinators, often better for a particular plant.), some wasps do this as well. Ants are often among the most numerous organisms in any particular environment, both by numbers and total weight, stereotypically swarming either to fight, or eat whatever type of food that particular species happens to like; some ant’s small size and ability to eat human based food makes them house pests.
Ant colonies are stereotypically underground, but they can make their homes by boring into other materials also. Bees are known for the honeycombs, in a kept beehive these are found on plates, but in the wild will squeeze into any reasonable spaces, bee infestations in walls of houses or trees or the ground are known. Social wasp hives are often paper or a similar hard material, some hardened by wasp saliva. Like beehives, they have individual cells where various things happen. Unlike termites, hive queens are a thing in these animals, one (sometimes a few) large reproducing female lays the eggs, small male drones act as mates and that’s about it. In ants, queens and drones are the only members that may have wings. Other specializations depend on the species, workers and fighters are common, sometimes specialized workers exist that, say, take care of larvae, and sometimes different types of fighters can be hatched.
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Strepsiptera
A group of parasitic insects with an unusual way of living. Females live inside another insect, creating a structure to block host defenses, and feeding off body fluids. Males are free flying, and livie very short lives that consist of finding a female and mating. Larvae are raised partly inside the female, before leaving and going after a new host. Hosts are, understandably, often hurt badly by this infestation.
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Beetles, or Coleoptera
Tropes:
Japanese Beetle Brothers
Tough Beetles
Scarab Power
Thunder Beetle
“What have you learned about God?’ “He had an inordinate fondness for beetles” is the quote, for beetles are the insect group with the most species. Fireflies, scarabs, and ladybugs might be the best known, some other fun ones are the goliath beetle, heaviest insect, and the bombardier beetle, which can spray hot material at attackers. With such a wide variety of species, beetles as a whole are not surprisingly found in just about all land environments, with species specialized to eat just about anything possible.
The main characteristic of beetles is the wing covering: most insects have 4 wings, in beetles two of these have developed into the shell we’re all familiar with, covering the other pair when folded, and providing extra protection to the animal when not flying, also protection for wings in tight spaces.
A lot of crop pests are beetles, such as Boll weevils for cotton, or a number of tree boring beetles that infest cities or forests occasionally.
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Neuropterida
Tropes:
Antlion Monster
Lacewings, dobsonflies, snakeflies, and others, but the one mentioned in this trope is the antlion, an ant eating (where the name comes from) insect known for the ambushes it carries out. Many of this group have large wings that don’t fit particularly well on their bodies. Contains a number of predators.
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Butterflies and Moths, or Lepidoptera
Tropes:
Butterfly of Death and Rebirth
Butterfly of Transformation
Moth Menace
Macabre Moth Motif
Pretty Butterflies
The plain caterpillar becomes a beautiful butterfly. Or a not so beautiful moth, but that’s a less common story. Lepidoptera means “scaled wing”, these scales are what let the animals have the wing patterns they are known for, and their body is also covered with these. Most are herbivores of some sort, many are pollinators, and they live in most places in the world, but many moths eat random bits of debris, including the ones that hang out in houses and eat bits of clothing or other things you really don’t want them eating. The Hungry Hungry Caterpillar gets the idea right, like many larvae lepidopteran ones will eat lots of plants, before going pupa and adult forms, this eating can cause major problems with crops.
The wing pattern can be used for camouflage, bright colors warning of poison, looking like larger animals, or nothing at all.
Silkworms are a part of this group, they are a type of moth larva.
Caddisflies, or Trichoptera
These live most of their lives as larvae in freshwater, adults live for a much shorter time. The larvae are important animals where they live, eating a variety of foods and being food themselves for lots of fish and other animals.
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Flies, or Diptera
Tropes:
Flies Are Evil
Messy Maggots
Super Fly Reflexes
Another “true” group (see true bugs), officially known as the true flies. These have a bad reputation among people, thanks to mosquitos biting and spreading diseases, annoying houseflies and fruit flies and such, maggots, and several species whose larvae live in people and animal’s skin, causing problems in the larger animal, this is called myiasis or being fly-blown.
Flies have just one pair of wings unlike other insect’s double set, and with this pair plus good senses are very agile in the air, thus swatting a fly as a show of reflexes. Fly’s dodging comes heavily from detecting air currents, which solid objects such as a hand generate more of than flexible or perforated objects, thus flyswatter designs with lots of holes. Pieces of cloth can also do the job if flicked or whipped in the right way.
Fly larvae eat lots of types of foods, but detritus and dead things are most common, thus many types of maggots. A few of these maggots can be used medically, sterilized (very important!) members of some species preferentially go for dead tissue while leaving living tissue mostly alone, and also have bacteria killing chemicals: these can remove dead tissue and reduce infections in some ulcers or other difficult to heal wounds. A lot of adult flies have sucking mouthparts, which can feed off of liquid as in fruit flies, or, as with mosquitos, suck blood. Thanks to fast reproduction and being well studied, some fruit flies are used as lab animals.
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Scorpionflies, or Mecoptera
Named for the end of theri abdomen which looks like, surprisingly enough, a scorpion tail. These look like the other group of flies, but instead live in moist areas and eat dead animals, or other random bits of what is available.
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Fleas, or siphonaptera
Tropes:
Flea Episode
Ntice the a-ptera, so no wings on these. Fleas are flightless, but are known for jumping seemingly absurdly high compared to their body size, another example of Strong Ant style Square-Cube Law in action. All of them live on mammals or birds, sucking blood. Larvae also live on skin, eating dead skin cells and other debris that accumulates. Like many blood suckers, they are known to transmit diseases (bubonic plague/black death being the most well known.)
Spiralia
Animals whose early embryos divide in a spiral way, in other words, as the fertilized egg cell divides, the resulting cells make a spiral pattern. The other group of protostomes.
Microscopic, mainly water living animals, with unusual hard mouthparts. Some are attached, some float, and some swim, eating up bits of debris and small animals. The mouthparts open into a brisly looking front looking something like a vase, the rest of the body can have a variety of shapes, though always symmetrical, such as conelike with a tail, or cup like.
The most well known of these are probably tapeworms, among many parasites, lots of flatworms are free living as well, living in water or moist environments. The free living types have a variety of lifestyles, some parasites do the bloodsucking thing, some like tapeworms and flukes live inside hosts, feeding off surrounding fluids.
Tropes:
Instant Leech: Just Fall in Water!
Pitiful Worms
Worm in an Apple
(Neither technically have to be annelids, but you probably imagine something earthworm like))
Earthworms and leeches, among others. Includes a number of deep sea worms, attached to the seafloor or nutrient sources such as sunken whale corpses or hydrothermal vents (including the red and white tubeworm pictures), polychaetes are the main group of these. Annelids grow in segments, each segment repeats some organs with a similar structure, with blood vessels, nerves, a digestive system running through, and similar connecting organs running through; they were once united with arthropods for this reason, but molecular analyses don't support this. Most have little hairlike things off the sides to help them move. The larger than normal ring you see on earthworms is a reproductive section, used to store eggs.
Both ocean and earthworms often burrow into the ground, allowing circulation into seabeds and soils and improving their quality, in addition to the usual feeding roles. Decomposers/debris eaters are the most common, some are predators or herbivores. Leeches are known for bloodsucking, though many are predators also.
While some of this group can regrow from cut up pieces, earthworms aren’t among them. Most whose reproduction is known take care of eggs, providing nutrients and/or protecting them in some way.
Tropes:
Mollusk Tropes
Original animals with shells, though some groups have lost these. Mollusc bod shapes have a structure called a foot, which you can see in their names (-pod = foot), a general muscular structure that has evolved into a number of roles, a scraping tongue with hard minerals in it called a radula, and a mantle that grows a shell in animals with one.
Mollusc groups include:
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Squid, Octopuses, Cuttlefish, and Nautiluses, or Cephalopods
Tropes:
Cephalopod Index
Tentacles and jets are the main features of this group, they are also known for high intelligence compared to other animals, especially octopuses. Tentacles, the “foot” on cephalopods, can be used for lots of purposes, their main use is holding prey while the animal eats it, squid arms are longer versions used mainly to reach out and grab prey. Hooks or suckers found on tentacles improve their ability to grab things. Jetting is a unique way cephalopods use to move, they expand their mantle (the “main body” of the organism, if you are looking at a whole one) to draw in water, then squirt it through a funnel. This does take a lot of energy to move fast, so most use fins or general swimming for slower movement, orwalking/crawling with tentacles as many octopuses do, while saving the jet for drifting movement or for fast bursts to escape or catch things. The squid/Octopus/Cuttlefish branch is known for intelligence, nautilus’s many have this as well but haven’t been checked as much. Also unusual to cephalopods are vertebrate like eyes, with a lens, single eyeball, retina, and such, though the retinal nerves are arranged differently.
Shelllessness is a recent thing in prehistoric terms, most cephalopods until a bit after the cretaceous extinction had shells, a group called belemnites is the ancestor to the current soft bodied forms. A bit of hard structure remains in a “gladius”, a bit of cartilage found in squid, and a small cuttlebone made of mineral in cuttlefish. The reason for shell loss is not known, sonar using whales seeing shells far more easily than soft bodies, or an increase in agility against predators that could get through shells, have been proposed. Nautilus ancestors obvious avoided these problems. The skin of the shellless cephalopods at some point evolved fast color changes and texture mimicry, you can find videos of amazing octopus camouflage in particular on youtube.
Nautiluses and octopuses mainly live near the seabed, octopuses commonly eat shelled organisms but can attack other types occasionally, Nautiluses scavenge and sometimes grab live animals when they can. Squid live up in the water column, grabbing larger animals to eat, and themselves are common food source for larger animals. Cuttlefish also tend to stay near the seafloor, going for shrimp and crabs mostly but eating what they can catch. Food is eaten with a cartilage beak (yes, really, it is shaped like a bird beak) and radula scraping.
Nautiluses tend to reproduce slowly and live long, while the other cephalopods die on reproducing and grow very quickly.
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Clams and Mussels, or Bivalves
You probably think of these as food, but of course there are lots of types that aren’t commonly eaten. In these creatures, the mantle secretes a two part shell, with a muscle and ligament attached to open or close the shell as needed, and the other usual organs. Most are filter feeders, and can be used to clarify water thanks to said feeding. Reproduction involves releasing sperm into the water, some species release eggs as well which quickly fertilize and hatch, in others females draw in sperm to fertilize their eggs.
While these organisms are known as stay in place filter feeders, most are able to swim using their shells, or burrow into the seabed using shell and foot movement and using a siphon to draw in water for feeding.
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Monoplacophorans
Rare, sea floor organisms, these have a cone shell with the body underneath, and are assumed to resemble the earliest molluscs the best. They had been assumed extinct until organisms were found in the 20th century.
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Snails and slugs, or Gastropods
One should not underestimate the humble snail. Cephalopods get attention for cleverness, bivalves and squid by food lovers, but snails have their own charms. (including food lovers, yes.) They are among the more species rich of animal groups, and live in just about every environment on earth from deep sea to desert, including several moves from water to land. Land snails are mostly plant eaters, maybe not surprising for s stereotypically slow animal, while water snails eat most types of food.
Snails are known for coiled shells, but some like limpets have flatter versions, or coils with a very small coil and large entrance, if you’ve ever collected seashells you will know the different shapes. Their main weird bits of anatomy are asymmetrical organs, plus a process called torsion that during development twists the rear section around, making some tail organs near their heads instead. The foot is used for movement, on land snails it gives off mucus to help the snail move. Stalks on some snails carry eyes and the ability to smell. The mantle creates the snail’s shell. Losing a shell has happened many times, creating different kinds of slugs.
Resembling clams, but with a different ancestry and different shell anatomy, with the two halves acting more like a mouth than side structures as in clams. The creatures attach to the sea floor using a long structure, and the shell either hangs in water, or extends out of a burrow for feeding. Larvae tend to resemble small adults, but swimming in the water.
Brachiopods are rarer today than in the past, they were the main clam type animal at points in prehistory, but clams have generally replaced them for various reasons as time went on.
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A small group of worms, they live on seafloor, stand upright, and use a web looking structure on top to catch food.
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A couple groups are usually included, with some different internal anatomy, but scientists aren’t sure if they are completely related. They are written as one group for simplicity.
Bryozoans are another group of attached, filter feeding ocean animals, with a variety of forms from stalklike to kelplike to others, the name means “moss animals” and some do look like this. Reproduction releases sperm that are either captured by bryozoans acting as females (in most species individuals can switch between the two) or fertilize free released egg. These hatch into mobile larvae that filter feed and/or rely on yolk until they find a place to settle down.
A group of usually very thin worms, mostly living in sediment, most act as predators, using a proboscis to capture prey. They are among worms that can be broken into pieces which regrow into complete animals. They don’t have a heart or blood vessels, instead using a tube network to move body fluids around, acting to both circulate nutrients and control movement of the animal.
Deuterostomes
The rear end of the digestive system forms first, this is the key characteristic of this group. Otherwise it is a typical mix of creatures, with fewer species than each of the protostome branches. Tunicates, vertebrates, and lancelets are all classified as chordates, considered a group equivalent to arthropod or annelid or such, but as they are very different creatures this useful notes has kept things separate.
Starfish, sea lilies, sea cucumbers, sea urchins among others. Minerals just underneath the skin surface provide some protection, while a water tube system provides movement. Some echinoderms, including the original ones, are attached filter feeders, sea cucumbers, sea urchins, and starfish can move slowly and eat things on the seafloor, mostly mussel and clam type organisms for starfish, sea cucumbers generally scavenge/eat debris in sediment, sea urchins mainly eat plants but can grab animals sometimes also. Movement is done by tube feet, which move water water pressure to move things around, some sea susumbers crawl instead like worms.
Echinoderm larvae are wormlike, but adults of most types except sea cucumbers are not, instead showing 5 side symmetry as is obvious in starfish (sea cucumbers have both, looking wormlike but also having 5 side symmetry circling around the cylinder).
Sea cumbers and starfish are known for being able to throw out organs, sea cumbuers in defense, starfish can extrude their stomaches to digest things outside the body, useful for getting into clamshells.
Acorn worms and pterobranches. Acorn worms are wormlike, but have an unusualanatomy where some organs are in a large lobe in front, as a result they look something like earthworms with a collar, smaller front, and larger back, but the mouth is actually on the collar. These draw water through their mouths and filter feed it, releasing it from gills. The mostly live burrowed in the seabed.
Pterobranches have a similar body shape, but have lots of tentavles/feathery things sticking out the top to help catch debris. They are closely related to extinct grapholites, described in Prehistoric Life.
A small group of wormlike animals, mostly seabed burrowing filter feeders. They resemble tunicate larvae and early vertebrates, with a nerve cord down the back, plus a cartilage rod. Though they have small fins that help swim, they mostly burrow into seabed with their mouths sticking out, filter feeding.
If you go back before other apes, before elephants, before birds, frogs, sharks, and other fish, your closest relatives are…some coral looking tubular filter feeding things. Yes, really. The resemblance is in lancelet like larvae, with a nerve cord and cartilage rod down the back, but as they become adults they lose these features.
Most tunicates (sea squirts) are attached filter feeders, forming a roughly tubelike shape where water can flow through and be filtered. Some (salps) are free floating, often transparent, and act as predators. A few called larvaceans don’t change much from the larval form, instead adapting the swimming tail to help catch debris.
Vertebrates are a relatively successful group as animals go, members live in just about every environment, including being one of two groups with flyers, and the group has more species than most other types of animals. However, vertebrates get disproportionate attention in media, probably for a couple reasons: 1. Most media is made by and for one of their number and 2. Vertebrates get much larger than other animals. For prehistoric life, add that bones and teeth fossilize well. Combine these, and most animals you interact directly with/notice will be vertebrates, with media reflecting this.
The big defining feature is a backbone, in most species extended into a full internal skeleton of cartilage or bone. Other features include adaptive immunity, where an immune system can change a response and “learn” to respond to different invaders, allowing vaccines in medicine, hemoglobin in blood, and eyes with a single lense and ball, with a retina on the back, a structure only shared with cephalopods with some minor differences. The skeleton is almost certainly the main reason vertebrates can get so big, it provides support that doesn’t exist in soft bodied animals, and can grow continuously unlike arthropod exoskeletons which must be molted. The skeleton also attaches to muscles and includes limbs, allowing walking, flying, and manipulating things. Common to chordates in general (lancelets and tunicates as well) but not as obvious in those others, are tails extending past the anus, other animals with back sections like scorpions have extended abdomens instead.
Vertebrate evolution is a great example of Early-Installment Weirdness, the first creatures were very small lancelet like animals, if they’d stayed like this an alternate intelligent species might call them “fin worms” or "chord worms" or similar. Instead, over tens of millions of years, they got bigger, full skeletons, fins including the classic fishtail, jaws, better senses and bone evolved, plus possibly some soft features that don’t fossilize; at around 400 million years ago some of the hugely diverse fish became the biggest things in the ocean and some others were able to walk onto land. Most of PrehistoricLife describes vertebrates, you can check out more details there. (Or The Other Wiki, or books, etc.)
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Hagfish and Lampreys, or Cyclostomes
Tropes:
Lamprey Mouth
Most vertebrates have jaws, hagfish and lampreys are the exception. Cyclostome roughly means “circle mouth”, a good description of the lamprey mouth. Coth are relatively low metabolism animals, hangifhs tend to scavenge, though diets are somewhat flexible and they occasionally grab other meat, lampreys have some bloodsucking parasite species, some species that filter feed as larvae and die shortly after reproducing as adults. Both are eel like, and hagfish even lack backbone that defines vertebrates, though like shellless molluscs other evidence points to them as descendents of the group.
Hagfish are known for making lots and lots of slime, partly as a defense against predators, and their skin makes good leather. Lampreys are sometimes eaten, though have few other uses.
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Sharks and Rays, or Cartilaginous Fish
Tropes:
Threatening Shark
Also includes things called ratfish and chimeras. These have a cartilage skeleton, not as hard as bone but lighter, and some muscles also attach to skin. On that skin are toothlike scales, helping reduce friction and providing some protection. Most cartilaginous fish fertilize internally instead of releasing eggs and sperm into water, some hold eggs in their body to hatch and a few give live birth. This is a problem with pollution, overfishing, accidental killing, and other such issues, as these populations take time to recover.
Most of these fish are meat eaters or filter feeders. Some standouts besides the famous meat eaters are whale sharks, the biggest fish around today, basking sharks which are also quite big. Basking shark’s weird appearance alive and as a decomposing corpse is probably responsible for lots of sea monster sightings. Sleeper and Greenland sharks are deep sea animals, with slow metabolisms and very long lived.
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Ray Finned Fish
Tropes:
These Tropes Are Fishy
About half of vertebrate species are in this group, if it wasn’t mentioned in another group of fish in this useful notes, it goes here. Ray finned fish have a few traits, a bony skeleton with relatively thin fin bones, and a swim bladder, a bag of gas the fish can inflate or deflate to control buoyancy. Fish are in every water environment on the planet, eating anything it is possible to eat, ranging in size from several ton sunfish to dwarf minnows of a few millimeters long.
Food lovers and makers may know about oily vs. non oily fish: the oil in the fish helps the fish float and stores energy, useful for fish that must swim long distances. Less intense swimmers don’t need this oil and become whitefish. (There are some weird ones like fatty catfish, of course.) Caviar is the eggs of certain species, most bony fish reproduce by putting lots of eggs and sperm into the water to fertilize, though in a large group there are always exceptions.
Bony fish have a variety of shapes, most with the classic fish form, but flat flounders and flatfish exist, the sunfish mentioned above are also rounded and flattened compared to the usual shape. Some, such as sturgeons and gars, have partial cartilage skeletons. Most modern fish are in the “Teleosti” group, with thinner scales and more agile swimming.
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Lobe Finned Fish
There are only a few species of these, lungfish and ceolacanths. Coelacanths are a famous animal thought extinct, but rediscovered in the 20th century, extinct relatives had been found as fossils from only long before. Lungfish live in some places in the southern hemisphere.
The main characteristic of lobe finned fish is thick fins, as the name suggests. They also have lungs, as the name lungfish suggest, though in Coelecanths this has evolved into an oil and air filled organ that helps flotation. Lungs most likely evolved first and later some became swim bladders in ray finned fish. There are a few other random differences with ray finned fish as well. Lungfish tend to live in rivers and can go dormant when they dry out, while coelecanths drift and swim through water and grab fish.\\
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Amphibians
Tropes:
Reptile and Amphibian Tropes
It isn’t known if modern amphibians have a common ancestor different than land vertebrates different theories of ancestry go either way, but they make a useful group for this useful notes.
Frogs, salamanders/newts, and small snakelike creatures called caecilians are the modern amphibians. They all must lay eggs in water, and have relatively thin skin that easily absorbs or loses water and other materials in it. This makes amphibians particularly sensitive to pollution, pictures of mutated frogs with extra legs or arms were a common news story in the 1990’s about pollution. The thin skin also means amphibians can absorb oxygen through it in addition to through lungs, and prevents them from living in salt water, so no amphibian versions of whales or plesiosaurs or penguins or such. Frog legs have specially evolved for jumping, which they can do for surprisingly long distances.
Frogs and salamanders tend to eat whatever small animals they can catch, sometimes using a tongue to catch food. Caecilians are highly specialized for burrowing, and seem to eat whatever they find underground.
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The Great Reptile Mix-Up
Land vertebrates that don’t need to lay eggs in water, or at least had an ancestor with this quality, are called amniotes. Some have returned to water, but these are either live birthing in some way, possibly keeping eggs inside the body, or return to land for reproduction. Includes mammals, birds, and rept…actually, we run into some issues here.
No group has been more messed with by phylogenetic classification than “reptiles”. When the original linnean system was created, land vertebrates naturally divided into furry warm blooded animals with milk (mammals), feathered warm blooded animals with wings (birds), thin skinned cold blooded animals laying eggs in water (amphibians), and cold blooded animals with drying resistant eggs often with scales, shells, or osteoderms (reptiles). This system still makes sense for vets and zookeepers and other people working with modern animals. However, it is not phylogenetic, birds and crocodiles have a common ancestry that lizards don’t, turtles may be associated with this group also.
Prehistoric animals mess this up even further. Because of various bone features, and later because cold blooded but laying eggs out of water was a stage modern amniotes clearly went through, but the animals didn’t have mammal or bird features, lots of prehistoric creatures were called “reptiles”, such as dimetrodon, therapsids (“mammal like reptiles”), dinosaurs, pterosaurs, large sea creatures, and many others. The whateversaurus tropes exists for this reason, -saurus mean lizard, and is a common suffix used for these animals. However, the phylogenetic problem still exists, lots of creatures will be more closely related to birds than lizards, probably also turtles, and “mammal like reptiles’ are closer to mammals than anything else, and equally related to birds and modern reptile groups. Classifying them by traits doesn’t work well either based on what we know, cold blooded lizards, live birthing possibly warm blooded ichthyosaurs, high metabolism fur like thing covered pterosaurs, feathered dinosaurs, turtles, and somewhat more active crocodile ancestors would not get classified into one group while leaving out birds and mammals, if someone were doing it from scratch today.
So, though Prehistoric Life uses the term reptile in this older sense, the official classification term is now used for diapsids, and this useful notes will describe groups separately.
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Mammals and other synapsids
Tropes:
Mammal Tropes
Amniotes split into synapsids and sauropsids early on, the groups are based on molecular evidence, and fossils where they have different numbers of holes in a skull for lightening and provided room for muscles. Synapsids have an up and down history, the first land vertebrates to get really big, than replace by the other ‘psids for a couple hundred million years over a couple mass extinctions, than with a third mass extinction became the big animals again. And at least one of them is reading this right now.\\
Mammals are probably the most tropperiffic group of animals, even if you don’t cheat and leave out all the society, culture, technology, romance, etc. tropes. They make out our livestock, pets, several pests, and the big wild animals we stay away from, or smaller wild animals we watch do things. Mammals are defined by milk, fur (at least in ancestors), and some skull structures such as a different ear and jaw shape. Most have live births, in marsupials like kangaroos to a small infant that crawls into a pouch and grows further, in placentals into a more or less partly grown baby that can survive with some care outside. Milk gives us dairy products and breastfeeding, it must supply lots of nutrients and energy, and will therefore be fatty,...have lots of nutrients, including protein, and be a good consistency for babies to drink. Milk production and live birth means mammals provide good parental care by animal standard, definitely no ‘release reproductive cells into water and hope for the best” as lots of other groups do, or “die after reproducing”. Fur is useful as insulation, it also protects from sun and provides color for displays.
By species, rodents and bats are the biggest groups, including a number of pests, or just annoyances as with bats in a house. Mammals include eaters of most types of foods, you probably know roughly which groups eat what. Insect eating is more common in smaller animals, larger ones tends towards larger prey or plants or a mix. Grass eaters are common livestock, a very useful quality as grass is one of the few foods people don’t handle well, allowing the use of an extra resource. The largest animals around today are a group of baleen whales, as described in the “size” section of this useful notes.
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Turtles
Tropes
Index in a Half Shell
Turtle ancestry is tricky, they for a long time seemingly showed up fully formed in the fossil record, and don’t have several features scientists normally use to classify land vertebrates, best guess was they were primitive/early diverging branch on the amniote . Today, molecular studies mostly point to the bird + crocodile group as close relatives, but a few point to lizards being closer.
Whatever their ancestry, turtles are obviously defined by their shell, created of bone or a thick leathery material. This shell obviously provides protection, but also limits movement, limits space for organs, including space to breathe, and requires nutrients to grow. In a more intimate issue, turtle penises must be really long to reach between the mating couple. Most of these problems are less extreme in water, fins can provide propulsion for example, and turtle lineages have evolved from land to water and back many times. Turtles also lack teeth, having beaks instead.
Most turtles eat whatever is available, with land turtles eating more plants and sea turtles eating more meat, as might be expected from how easy walking is vs. swimming. Among the more specialized turtles are leatherbacks, eating jellyfish. Metabolism is generally slow, apart from some sea turtles that maintain a high temperature partly through physical activity. The combination of slow metabolism and protection is probably what allows turtles to live such a long time, close to 200 year lifespans have been measured in a few.
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Crocodilians
Tropes:
Never Smile at a Crocodile
Sewer Gator
Green Gators
The crocodile group, pseudosuchians, has a lot of interesting prehistoric members which are worth reading about, but the only remaining members are the alligators and crocodiles (and gharials and caimans) we all have heard of today. All of these mainly live in rivers, eating fish, land animals they can ambush, or a mix of the three. Crocodiles and alligators are differentiated by random bits of anatomy, caimans also have some of these and are usually smaller, and gharials have a very thin snout that trades ability to catch land prey for fast closing to catch fish.
While fish are a big part of lots of crocodile’s diets, catching land animals is what they are famous for, and they have a lot of anatomy to do it. Crocodiles are famous for a very strong bite, and for their ridiculously weak jaw opening muscles, this bite is used to hold animals in water to drown them, followed by the crocodile eating, sometimes rolling to separate bits of meat to swallow. They are adapted to swim low in water, eyes and nostrils on top of the head. Greyish slight greenish (exaggerated by pictures) color adds camouflage, resembling logs or natural river color.
Crocodiles have a lot of anatomy suggesting a more active ancestry: 4 chambered hearts, found in birds and mammals, plus the ability to stand and walk upright and some can even gallop in a way, though they can't do this for long without tiring. See prehistoric life for more details.
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Birds and other Avemetatarsalia
Tropes:
Bird Tropes
Dinosaur Tropes
Terror-dactyl
Dinosaurs
Another highly troperiffic group, including the Pterosaur, a bunch of large prehistoric land animals you played with as a kid, and modern birds are no slouch either, such as the majestic Eagle, the Circling Vultures waiting for something to die, Chicken which all meat tastes like, Clever Crows or Creepy Crows as fits your preference, and many others.
Birds today are known for being warm blooded, having beaks, and feathers most obviously. Other adaptations include air filled bones, very efficient lungs, and large breast muscles used for pulling the wings. All in some way contribute to flying, beaks and air filled bones lighten the animal, powerful chest muscles work the wings, high metabolism and efficient lungs give the bird energy to fly, a high power to weight for animals, feathers form the airfoil of the wings and keep birds warm to help maintain a high temperature. The lungs have a very different structure than what you are familiar with, instead of lungs that draw air in and out and absorb it, birds have a rigid organ than absorbs oxygen, and a couple systems of air sacs that expand and contract, all attached by tubes. These sac’s movement is timed so that fresh air is always flowing through the oxygen absorber, while stale air bypasses this organ and is exhaled directly. This helps birds handle much lower oxygen levels than almost all animals can handle, they can fly at altitudes where humans need extra air. Fligthless birds maintain these adaptations. The evolution of many of these features is a subject of interest for paleontologists, a lot show up in bird’s flying relatives Pterosaurs as well.
Different birds obviously eat a variety of foods and fill a variety of roles in their environment. Studies of how beak shapes relate to bird’s diets and lifestyles are a common example of evolution at work. Bird reproduction is by eggs, which most birds protect to keep a proper temperature. Parental care is common among birds as well.
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Lizards, Snakes, Tuatara, or Lepidosaurs
Tropes:
Hollywood Chameleons
Lovable Lizard
Malicious Monitor Lizard
Snake and Serpent Tropes
Probably the animal group that most resembles the earliest amniotes, though still evolved over time. Lepido- means scaled, and scales are a key feature of these animals. Lizards are the familiar four legged, long tailed things you know about, snakes are the scary animals with no legs you don’t want biting or constricting you, tuatara are a group found only in New Zealand that look roughly like lizards, though with a third eye on their head and some skeleton differences. The scales give some protection, and help with other skin functions. Snakes have evolved from within lizards, lizards and tuatara have a common ancestor. The scales are overlapping, and have to be shed, sometimes as a big piece shaped like the whole animal.
Lizards and tuatara tend to move in a sprawling posture, with legs splayed out to the side. This.takes less energy when standing up or laying down, but limits their movement as sprawling creates more side to side movement, plus constricts lizard lungs and interferes with breathing Some can run on two legs for a time. Snakes obviously crawl along the ground in a number of ways, either moving some stomach sections forward while pushing with others, or using a wide to side motion that pushes using the sideways sections.
As usual for this sort of large group, lizards fill a variety of ecological roles, eating a variety of diets. Most lay eggs to reproduce, but a few give live birth, and some females can reproduce without mating. Most have slow metabolism, a few have something in between birds and mammals and most lizards. Snakes are almost all carnivores, with rodents as the big source of prey. It is known that some are venomous, some people think all snakes have at least a little venom (though obviously not all dangerous to people), and are part of a large group with some venomous lizards. Constrictor snakes are the name for the ones that squeeze prey, mostly big muscular types for obvious reasons, the exact killing mechanism could be causing heart failure, or cutting off breath.
